.../EventSubscriber/DefaultExceptionSubscriber.php | 69 ++++++++--------------
 1 file changed, 24 insertions(+), 45 deletions(-)

diff --git a/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php b/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
index e58d980..5a21bf4 100644
--- a/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
+++ b/core/modules/serialization/src/EventSubscriber/DefaultExceptionSubscriber.php
@@ -4,9 +4,9 @@
 
 use Drupal\Core\EventSubscriber\HttpExceptionSubscriberBase;
 use Drupal\Core\Utility\Error;
-use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpException;
 use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
 use Symfony\Component\Serializer\SerializerInterface;
 
@@ -62,19 +62,19 @@ protected static function getPriority() {
    * {@inheritdoc}
    */
   public function onException(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    if (in_array($event->getRequest()->getRequestFormat(), $this->getHandledFormats(), TRUE)) {
+      if (!$exception instanceof HttpExceptionInterface) {
+        $error = Error::decodeException($exception);
+        $message = error_displayable($error)
+          ? sprintf('A fatal error occurred: %s', $exception->getMessage())
+          : 'A fatal error occurred.';
+        $event->setException(new HttpException(500, $message, $exception));
+      }
+    }
     parent::onException($event);
-
-    // If the exception was an HttpExceptionInterface exception, then
-    // propagation will already have stopped. If propagation has not yet stopped
-    // this must be a fatal error.
-    if (!$event->isPropagationStopped() && in_array($event->getRequest()->getRequestFormat(), $this->getHandledFormats(), TRUE)) {
-      $exception = $event->getException();
-      $error = Error::decodeException($exception);
-      $message = error_displayable($error)
-        ? sprintf('A fatal error occurred: %s', $exception->getMessage())
-        : 'A fatal error occurred.';
-
-      $event->setResponse($this->buildResponse($message, 500, [], $event->getRequest()));
+    if (!$event->isPropagationStopped()) {
+      $event->setException($exception);
     }
   }
 
@@ -87,13 +87,18 @@ public function onException(GetResponseForExceptionEvent $event) {
   public function on4xx(GetResponseForExceptionEvent $event) {
     /** @var \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface $exception */
     $exception = $event->getException();
+    $request = $event->getRequest();
 
-    $event->setResponse($this->buildResponse(
-      $exception->getMessage(),
-      $exception->getStatusCode(),
-      $exception->getHeaders(),
-      $event->getRequest()
-    ));
+    $format = $request->getRequestFormat();
+    $content = ['message' => $event->getException()->getMessage()];
+    $encoded_content = $this->serializer->serialize($content, $format);
+    $headers = $exception->getHeaders();
+
+    // Add the MIME type from the request to send back in the header.
+    $headers['Content-Type'] = $request->getMimeType($format);
+
+    $response = new Response($encoded_content, $exception->getStatusCode(), $headers);
+    $event->setResponse($response);
   }
 
   /**
@@ -106,30 +111,4 @@ public function on500(GetResponseForExceptionEvent $event) {
     return $this->on4xx($event);
   }
 
-  /**
-   * Builds an error response.
-   *
-   * @param string $message
-   *   The message to use in the error response.
-   * @param int $status_code
-   *   The response status code.
-   * @param array $headers
-   *   An array of response headers.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   The current request.
-   *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   The error response to send.
-   */
-  protected function buildResponse($message, $status_code, $headers, Request $request) {
-    $format = $request->getRequestFormat();
-    $content = ['message' => $message];
-    $encoded_content = $this->serializer->serialize($content, $format);
-
-    // Add the MIME type from the request to send back in the header.
-    $headers['Content-Type'] = $request->getMimeType($format);
-
-    return new Response($encoded_content, $status_code, $headers);
-  }
-
 }