.../EventSubscriber/DefaultExceptionSubscriber.php | 55 ++++++++++++++++++----
 .../DefaultExceptionSubscriberTest.php             | 21 ++++++++-
 2 files changed, 66 insertions(+), 10 deletions(-)

diff --git a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
index fdc2385..4d3bffc 100644
--- a/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/lib/Drupal/Core/EventSubscriber/DefaultExceptionSubscriber.php
@@ -70,7 +70,7 @@ public function onException(GetResponseForExceptionEvent $event) {
     // Display the message if the current error reporting level allows this type
     // of message to be displayed, and unconditionally in update.php.
     $message = '';
-    if (error_displayable($error)) {
+    if ($this->isErrorDisplayable($error)) {
       // If error type is 'User notice' then treat it as debug information
       // instead of an error message.
       // @see debug()
@@ -78,16 +78,11 @@ public function onException(GetResponseForExceptionEvent $event) {
         $error['%type'] = 'Debug';
       }
 
-      // Attempt to reduce verbosity by removing DRUPAL_ROOT from the file path
-      // in the message. This does not happen for (false) security.
-      $root_length = strlen(DRUPAL_ROOT);
-      if (substr($error['%file'], 0, $root_length) == DRUPAL_ROOT) {
-        $error['%file'] = substr($error['%file'], $root_length + 1);
-      }
+      $error = $this->simplifyFileInError($error);
 
       unset($error['backtrace']);
 
-      if ($this->getErrorLevel() != ERROR_REPORTING_DISPLAY_VERBOSE) {
+      if (!$this->isErrorLevelVerbose()) {
         // Without verbose logging, use a simple message.
 
         // We call SafeMarkup::format directly here, rather than use t() since
@@ -138,4 +133,48 @@ public static function getSubscribedEvents() {
     return $events;
   }
 
+  /**
+   * Checks whether the error level is verbose or not.
+   *
+   * @return bool
+   */
+  protected function isErrorLevelVerbose() {
+    return $this->getErrorLevel() != ERROR_REPORTING_DISPLAY_VERBOSE;
+  }
+
+  /**
+   * Wrapper for error_displayable().
+   *
+   * @param $error
+   *   Optional error to examine for ERROR_REPORTING_DISPLAY_SOME.
+   *
+   * @return bool
+   *
+   * @see \error_displayable
+   */
+  protected function isErrorDisplayable($error) {
+    return error_displayable($error);
+  }
+
+  /**
+   * Attempts to reduce error verbosity in the error message's file path.
+   *
+   * Attempts to reduce verbosity by removing DRUPAL_ROOT from the file path in
+   * the message. This does not happen for (false) security.
+   *
+   * @param $error
+   *   Optional error to examine for ERROR_REPORTING_DISPLAY_SOME.
+   *
+   * @return
+   *   The updated $error.
+   */
+  protected function simplifyFileInError($error) {
+    // Attempt to reduce verbosity by removing DRUPAL_ROOT from the file path
+    // in the message. This does not happen for (false) security.
+    $root_length = strlen(DRUPAL_ROOT);
+    if (substr($error['%file'], 0, $root_length) == DRUPAL_ROOT) {
+      $error['%file'] = substr($error['%file'], $root_length + 1);
+    }
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php b/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php
index 279d830..a0a48a1 100644
--- a/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/EventSubscriber/DefaultExceptionSubscriberTest.php
@@ -26,12 +26,13 @@ public function testOnExceptionWithUnknownFormat() {
     $request = Request::create('/test?_format=bananas');
     $e = new MethodNotAllowedHttpException(['POST', 'PUT'], 'test message');
     $event = new GetResponseForExceptionEvent($kernel->reveal(), $request, 'GET', $e);
-    $subscriber = new DefaultExceptionSubscriber($config_factory);
+    $subscriber = new TestDefaultExceptionSubscriber($config_factory);
+    $subscriber->setStringTranslation($this->getStringTranslationStub());
     $subscriber->onException($event);
     $response = $event->getResponse();
 
     $this->assertInstanceOf(Response::class, $response);
-    $this->assertEquals('test message', $response->getContent());
+    $this->stringStartsWith('The website encountered an unexpected error. Please try again later.</br></br><em class="placeholder">Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException</em>: test message in ', $response->getContent());
     $this->assertEquals(405, $response->getStatusCode());
     $this->assertEquals('POST, PUT', $response->headers->get('Allow'));
     // Also check that that text/plain content type was added.
@@ -39,3 +40,19 @@ public function testOnExceptionWithUnknownFormat() {
   }
 
 }
+
+class TestDefaultExceptionSubscriber extends DefaultExceptionSubscriber {
+
+  protected function isErrorDisplayable($error) {
+    return TRUE;
+  }
+
+  protected function simplifyFileInError($error) {
+    return $error;
+  }
+
+  protected function isErrorLevelVerbose() {
+    return TRUE;
+  }
+
+}
\ No newline at end of file