diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 9849d43..2195ffe 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -488,10 +488,7 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
 
   // Use a default value if $message is not set.
   if (empty($message)) {
-    // The exception message is run through
-    // \Drupal\Component\Utility\SafeMarkup::checkPlain() by
-    // \Drupal\Core\Utility\Error:decodeException().
-    $message = '%type: !message in %function (line %line of %file).';
+    $message = '%type: @message in %function (line %line of %file).';
   }
 
   if ($link) {
diff --git a/core/includes/errors.inc b/core/includes/errors.inc
index 6933dc8..c41c688 100644
--- a/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -68,7 +68,7 @@ function _drupal_error_handler_real($error_level, $message, $filename, $line, $c
       '%type' => isset($types[$error_level]) ? $severity_msg : 'Unknown error',
       // The standard PHP error handler considers that the error messages
       // are HTML. We mimick this behavior here.
-      '!message' => Xss::filterAdmin($message),
+      '@message' => Xss::filterAdmin($message),
       '%function' => $caller['function'],
       '%file' => $caller['file'],
       '%line' => $caller['line'],
@@ -109,9 +109,9 @@ function error_displayable($error = NULL) {
  * Logs a PHP error or exception and displays an error page in fatal cases.
  *
  * @param $error
- *   An array with the following keys: %type, !message, %function, %file,
+ *   An array with the following keys: %type, @message, %function, %file,
  *   %line, severity_level, and backtrace. All the parameters are plain-text,
- *   with the exception of !message, which needs to be a safe HTML string, and
+ *   with the exception of @message, which needs to be an HTML string, and
  *   backtrace, which is a standard PHP backtrace.
  * @param $fatal
  *   TRUE if the error is fatal.
@@ -145,7 +145,7 @@ function _drupal_log_error($error, $fatal = FALSE) {
     // as it uniquely identifies each PHP error.
     static $number = 0;
     $assertion = array(
-      $error['!message'],
+      $error['@message'],
       $error['%type'],
       array(
         'function' => $error['%function'],
@@ -161,14 +161,14 @@ function _drupal_log_error($error, $fatal = FALSE) {
   // if there is an error while rebuilding the container or during the
   // installer.
   if (\Drupal::hasService('logger.factory')) {
-    \Drupal::logger('php')->log($error['severity_level'], '%type: !message in %function (line %line of %file).', $error);
+    \Drupal::logger('php')->log($error['severity_level'], '%type: @message in %function (line %line of %file).', $error);
   }
 
   if (PHP_SAPI === 'cli') {
     if ($fatal) {
       // When called from CLI, simply output a plain text message.
       // Should not translate the string to avoid errors producing more errors.
-      print html_entity_decode(strip_tags(format_string('%type: !message in %function (line %line of %file).', $error))). "\n";
+      print html_entity_decode(strip_tags(format_string('%type: @message in %function (line %line of %file).', $error))). "\n";
       exit;
     }
   }
@@ -178,7 +178,7 @@ function _drupal_log_error($error, $fatal = FALSE) {
       if (error_displayable($error)) {
         // When called from JavaScript, simply output the error message.
         // Should not translate the string to avoid errors producing more errors.
-        print format_string('%type: !message in %function (line %line of %file).', $error);
+        print format_string('%type: @message in %function (line %line of %file).', $error);
       }
       exit;
     }
@@ -205,12 +205,16 @@ function _drupal_log_error($error, $fatal = FALSE) {
           $error['%file'] = substr($error['%file'], $root_length + 1);
         }
       }
-      // Should not translate the string to avoid errors producing more errors.
-      $message = format_string('%type: !message in %function (line %line of %file).', $error);
 
       // Check if verbose error reporting is on.
       $error_level = _drupal_get_error_level();
 
+      // With verbose logging, we will append a backtrace. To keep our string
+      // safe, we need to include the <pre> tags inside the string to be
+      // formatted, and not included as part of the placeholder.
+      // We call SafeMarkup::format directly here, rather than use t() since
+      // we are in the middle of error handling, and we don't want t() to
+      // cause further errors.
       if ($error_level == ERROR_REPORTING_DISPLAY_VERBOSE) {
         // First trace is the error itself, already contained in the message.
         // While the second trace is the error source and also contained in the
@@ -218,11 +222,16 @@ function _drupal_log_error($error, $fatal = FALSE) {
         // once more in the backtrace.
         array_shift($backtrace);
         // Generate a backtrace containing only scalar argument values.
-        $message .= '<pre class="backtrace">' . Error::formatBacktrace($backtrace) . '</pre>';
+        $error['@backtrace'] = Error::formatBacktrace($backtrace);
+        $message = SafeMarkup::format('%type: @message in %function (line %line of %file). <pre class="backtrace">@backtrace</pre>', $error);
+      }
+      else {
+        $message = SafeMarkup::format('%type: @message in %function (line %line of %file).', $error);
       }
+
       if (\Drupal::hasService('session')) {
         // Message display is dependent on sessions being available.
-        drupal_set_message(SafeMarkup::set($message), $class, TRUE);
+        drupal_set_message($message, $class, TRUE);
       }
       else {
         print $message;
diff --git a/core/includes/update.inc b/core/includes/update.inc
index 9d4557f..e59abd0 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -191,7 +191,7 @@ function update_do_one($module, $number, $dependency_map, &$context) {
       // The exception message is run through
       // \Drupal\Component\Utility\SafeMarkup::checkPlain() by
       // \Drupal\Core\Utility\Error::decodeException().
-      $ret['#abort'] = array('success' => FALSE, 'query' => t('%type: !message in %function (line %line of %file).', $variables));
+      $ret['#abort'] = array('success' => FALSE, 'query' => t('%type: @message in %function (line %line of %file).', $variables));
     }
   }
 
@@ -239,10 +239,7 @@ function update_entity_definitions($module, $number, &$context) {
     watchdog_exception('update', $e);
     $variables = Error::decodeException($e);
     unset($variables['backtrace']);
-    // The exception message is run through
-    // \Drupal\Component\Utility\SafeMarkup::checkPlain() by
-    // \Drupal\Core\Utility\Error::decodeException().
-    $ret['#abort'] = array('success' => FALSE, 'query' => t('%type: !message in %function (line %line of %file).', $variables));
+    $ret['#abort'] = array('success' => FALSE, 'query' => t('%type: @message in %function (line %line of %file).', $variables));
     $context['results'][$module][$number] = $ret;
     $context['results']['#abort'][] = 'update_entity_definitions';
   }
diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php
index e3ec321..770b96f 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionHtmlSubscriber.php
@@ -161,7 +161,7 @@ protected function makeSubrequest(GetResponseForExceptionEvent $event, $url, $st
         // just log it. The DefaultExceptionSubscriber will catch the original
         // exception and handle it normally.
         $error = Error::decodeException($e);
-        $this->logger->log($error['severity_level'], '%type: !message in %function (line %line of %file).', $error);
+        $this->logger->log($error['severity_level'], '%type: @message in %function (line %line of %file).', $error);
       }
     }
   }
diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
index 883927c..7438a81 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -104,12 +104,18 @@ protected function onHtml(GetResponseForExceptionEvent $event) {
       if (substr($error['%file'], 0, $root_length) == DRUPAL_ROOT) {
         $error['%file'] = substr($error['%file'], $root_length + 1);
       }
-      // Do not translate the string to avoid errors producing more errors.
-      unset($error['backtrace']);
-      $message = SafeMarkup::format('%type: !message in %function (line %line of %file).', $error);
 
       // Check if verbose error reporting is on.
-      if ($this->getErrorLevel() == ERROR_REPORTING_DISPLAY_VERBOSE) {
+      $error_level = _drupal_get_error_level();
+      unset($error['backtrace']);
+
+      // With verbose logging, we will append a backtrace. To keep our string
+      // safe, we need to include the <pre> tags inside the string to be
+      // formatted, and not included as part of the placeholder.
+      // We call SafeMarkup::format directly here, rather than use t() since
+      // we are in the middle of error handling, and we don't want t() to
+      // cause further errors.
+      if ($error_level == ERROR_REPORTING_DISPLAY_VERBOSE) {
         $backtrace_exception = $exception;
         while ($backtrace_exception->getPrevious()) {
           $backtrace_exception = $backtrace_exception->getPrevious();
@@ -120,12 +126,16 @@ protected function onHtml(GetResponseForExceptionEvent $event) {
         // message, the message doesn't contain argument values, so we output it
         // once more in the backtrace.
         array_shift($backtrace);
-
-        // Generate a backtrace containing only scalar argument values. Make
-        // sure the backtrace is escaped as it can contain user submitted data.
-        $message .= '<pre class="backtrace">' . SafeMarkup::escape(Error::formatBacktrace($backtrace)) . '</pre>';
+        // Generate a backtrace containing only scalar argument values.
+        $error['@backtrace'] = Error::formatBacktrace($backtrace);
+        $message = SafeMarkup::format('%type: @message in %function (line %line of %file). <pre class="backtrace">@backtrace</pre>', $error);
+      }
+      else {
+        $message = SafeMarkup::format('%type: @message in %function (line %line of %file).', $error);
       }
-      drupal_set_message(SafeMarkup::set($message), $class, TRUE);
+
+      // Do not translate the string to avoid errors producing more errors.
+      drupal_set_message($message, $class, TRUE);
     }
 
     $content = $this->t('The website encountered an unexpected error. Please try again later.');
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php
index cb2fc31..6a2bd6c 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php
@@ -68,7 +68,7 @@ public function on404(GetResponseForExceptionEvent $event) {
   public function onError(GetResponseForExceptionEvent $event) {
     $exception = $event->getException();
     $error = Error::decodeException($exception);
-    $this->logger->get('php')->log($error['severity_level'], '%type: !message in %function (line %line of %file).', $error);
+    $this->logger->get('php')->log($error['severity_level'], '%type: @message in %function (line %line of %file).', $error);
 
     $is_critical = !$exception instanceof HttpExceptionInterface || $exception->getStatusCode() >= 500;
     if ($is_critical) {
diff --git a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
index d8f1a61..17c87a3 100644
--- a/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/ExceptionTestSiteSubscriber.php
@@ -51,7 +51,7 @@ public function on500(GetResponseForExceptionEvent $event) {
       // as it uniquely identifies each PHP error.
       static $number = 0;
       $assertion = array(
-        $error['!message'],
+        $error['@message'],
         $error['%type'],
         array(
           'function' => $error['%function'],
diff --git a/core/lib/Drupal/Core/Utility/Error.php b/core/lib/Drupal/Core/Utility/Error.php
index c72fba4..d150ec3 100644
--- a/core/lib/Drupal/Core/Utility/Error.php
+++ b/core/lib/Drupal/Core/Utility/Error.php
@@ -70,7 +70,7 @@ public static function decodeException($exception) {
       '%type' => get_class($exception),
       // The standard PHP exception handler considers that the exception message
       // is plain-text. We mimic this behavior here.
-      '!message' => SafeMarkup::checkPlain($message),
+      '@message' => $message,
       '%function' => $caller['function'],
       '%file' => $caller['file'],
       '%line' => $caller['line'],
@@ -94,15 +94,14 @@ public static function renderExceptionSafe($exception) {
     unset($decode['backtrace']);
     // Remove 'main()'.
     array_shift($backtrace);
+    $decode['@backtrace'] = $backtrace;
 
-    $output = SafeMarkup::format('%type: !message in %function (line %line of %file).', $decode);
     // Even though it is possible that this method is called on a public-facing
     // site, it is only called when the exception handler itself threw an
     // exception, which normally means that a code change caused the system to
     // no longer function correctly (as opposed to a user-triggered error), so
     // we assume that it is safe to include a verbose backtrace.
-    $output .= '<pre>' . static::formatBacktrace($backtrace) . '</pre>';
-    return SafeMarkup::set($output);
+    return SafeMarkup::format('%type: @message in %function (line %line of %file). <pre class="backtrace">@backtrace</pre>', $decode);
   }
 
   /**
diff --git a/core/modules/migrate/src/MigrateExecutable.php b/core/modules/migrate/src/MigrateExecutable.php
index 139a07d..3331c83 100644
--- a/core/modules/migrate/src/MigrateExecutable.php
+++ b/core/modules/migrate/src/MigrateExecutable.php
@@ -617,7 +617,7 @@ protected function getTimeElapsed() {
    */
   protected function handleException(\Exception $exception, $save = TRUE) {
     $result = Error::decodeException($exception);
-    $message = $result['!message'] . ' (' . $result['%file'] . ':' . $result['%line'] . ')';
+    $message = $result['@message'] . ' (' . $result['%file'] . ':' . $result['%line'] . ')';
     if ($save) {
       $this->saveMessage($message);
     }
diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index 9078d39..209e646 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -1396,8 +1396,8 @@ protected function exceptionHandler($exception) {
     // message through \Drupal\Component\Utility\SafeMarkup::checkPlain().
     $decoded_exception = Error::decodeException($exception);
     unset($decoded_exception['backtrace']);
-    $message = SafeMarkup::format('%type: !message in %function (line %line of %file). <pre class="backtrace">!backtrace</pre>', $decoded_exception + array(
-      '!backtrace' => Error::formatBacktrace($verbose_backtrace),
+    $message = SafeMarkup::format('%type: @message in %function (line %line of %file). <pre class="backtrace">@backtrace</pre>', $decoded_exception + array(
+      '@backtrace' => Error::formatBacktrace($verbose_backtrace),
     ));
     $this->error($message, 'Uncaught exception', Error::getLastCaller($backtrace));
   }
diff --git a/core/modules/system/src/Tests/System/ErrorHandlerTest.php b/core/modules/system/src/Tests/System/ErrorHandlerTest.php
index a42686b..cf00d62 100644
--- a/core/modules/system/src/Tests/System/ErrorHandlerTest.php
+++ b/core/modules/system/src/Tests/System/ErrorHandlerTest.php
@@ -30,32 +30,32 @@ function testErrorHandler() {
     $config = $this->config('system.logging');
     $error_notice = array(
       '%type' => 'Notice',
-      '!message' => 'Undefined variable: bananas',
+      '@message' => 'Undefined variable: bananas',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()',
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
     );
     $error_warning = array(
       '%type' => 'Warning',
-      '!message' => 'Division by zero',
+      '@message' => 'Division by zero',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()',
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
     );
     $error_user_notice = array(
       '%type' => 'User warning',
-      '!message' => 'Drupal is awesome',
+      '@message' => 'Drupal is awesome',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->generateWarnings()',
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
     );
     $fatal_error = array(
       '%type' => 'Recoverable fatal error',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->Drupal\error_test\Controller\{closure}()',
-      '!message' => 'Argument 1 passed to Drupal\error_test\Controller\ErrorTestController::Drupal\error_test\Controller\{closure}() must be of the type array, string given, called in ' . \Drupal::root() . '/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php on line 66 and defined',
+      '@message' => 'Argument 1 passed to Drupal\error_test\Controller\ErrorTestController::Drupal\error_test\Controller\{closure}() must be of the type array, string given, called in ' . \Drupal::root() . '/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php on line 66 and defined',
     );
     if (version_compare(PHP_VERSION, '7.0.0-dev') >= 0)  {
       // In PHP 7, instead of a recoverable fatal error we get a TypeException.
       $fatal_error['%type'] = 'TypeException';
       // The error message also changes in PHP 7.
-      $fatal_error['!message'] = 'Argument 1 passed to Drupal\error_test\Controller\ErrorTestController::Drupal\error_test\Controller\{closure}() must be of the type array, string given, called in ' . \Drupal::root() . '/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php on line 66';
+      $fatal_error['@message'] = 'Argument 1 passed to Drupal\error_test\Controller\ErrorTestController::Drupal\error_test\Controller\{closure}() must be of the type array, string given, called in ' . \Drupal::root() . '/core/modules/system/tests/modules/error_test/src/Controller/ErrorTestController.php on line 66';
     }
 
     // Set error reporting to display verbose notices.
@@ -123,21 +123,21 @@ function testExceptionHandler() {
 
     $error_exception = array(
       '%type' => 'Exception',
-      '!message' => 'Drupal is awesome',
+      '@message' => 'Drupal is awesome',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->triggerException()',
       '%line' => 56,
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
     );
     $error_pdo_exception = array(
       '%type' => 'DatabaseExceptionWrapper',
-      '!message' => 'SELECT * FROM bananas_are_awesome',
+      '@message' => 'SELECT * FROM bananas_are_awesome',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->triggerPDOException()',
       '%line' => 64,
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
     );
     $error_renderer_exception = array(
       '%type' => 'Exception',
-      '!message' => 'This is an exception that occurs during rendering',
+      '@message' => 'This is an exception that occurs during rendering',
       '%function' => 'Drupal\error_test\Controller\ErrorTestController->Drupal\error_test\Controller\{closure}()',
       '%line' => 82,
       '%file' => drupal_get_path('module', 'error_test') . '/error_test.module',
@@ -152,9 +152,9 @@ function testExceptionHandler() {
     // We cannot use assertErrorMessage() since the exact error reported
     // varies from database to database. Check that the SQL string is displayed.
     $this->assertText($error_pdo_exception['%type'], format_string('Found %type in error page.', $error_pdo_exception));
-    $this->assertText($error_pdo_exception['!message'], format_string('Found !message in error page.', $error_pdo_exception));
+    $this->assertText($error_pdo_exception['@message'], format_string('Found @message in error page.', $error_pdo_exception));
     $error_details = format_string('in %function (line ', $error_pdo_exception);
-    $this->assertRaw($error_details, format_string("Found '!message' in error page.", array('!message' => $error_details)));
+    $this->assertRaw($error_details, format_string("Found '@message' in error page.", array('@message' => $error_details)));
 
     $this->drupalGet('error-test/trigger-renderer-exception');
     $this->assertTrue(strpos($this->drupalGetHeader(':status'), '500 Service unavailable (with message)'), 'Received expected HTTP status line.');
@@ -180,15 +180,16 @@ function testExceptionHandler() {
    * Helper function: assert that the error message is found.
    */
   function assertErrorMessage(array $error) {
-    $message = t('%type: !message in %function (line ', $error);
-    $this->assertRaw($message, format_string('Found error message: !message.', array('!message' => $message)));
+    $message = t('%type: @message in %function (line ', $error);
+    debug($message);
+    $this->assertRaw($message, format_string('Found error message: @message.', array('@message' => $message)));
   }
 
   /**
    * Helper function: assert that the error message is not found.
    */
   function assertNoErrorMessage(array $error) {
-    $message = t('%type: !message in %function (line ', $error);
-    $this->assertNoRaw($message, format_string('Did not find error message: !message.', array('!message' => $message)));
+    $message = t('%type: @message in %function (line ', $error);
+    $this->assertNoRaw($message, format_string('Did not find error message: @message.', array('@message' => $message)));
   }
 }
diff --git a/sites/default/default.services.yml b/sites/default/default.services.yml
index ecb1c43..a9072ca 100644
--- a/sites/default/default.services.yml
+++ b/sites/default/default.services.yml
@@ -25,9 +25,9 @@ parameters:
     cookie_lifetime: 2000000
     #
     # Drupal automatically generates a unique session cookie name based on the
-    # full domain name used to access the site. This mechanism is sufficient
-    # for most use-cases, including multi-site deployments. However, if it is
-    # desired that a session can be reused across different subdomains, the
+    # full domain name used to access the site. This mechanism is sufficent for
+    # most use-cases, including multi-site deployments. However, if it is
+    # desired that a session can be reused accross different subdomains, the
     # cookie domain needs to be set to the shared base domain. Doing so assures
     # that users remain logged in as they cross between various subdomains.
     # To maximize compatibility and normalize the behavior across user agents,