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diff --git a/web/core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php b/web/core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php
new file mode 100644
index 000000000..4317ff08c
--- /dev/null
+++ b/web/core/lib/Drupal/Core/EventSubscriber/FinalExceptionSubscriber.php
@@ -0,0 +1,195 @@
+<?php
+
+namespace Drupal\Core\EventSubscriber;
+
+use Drupal\Component\Utility\SafeMarkup;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Utility\Error;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Last-chance handler for exceptions: the final exception subscriber.
+ *
+ * This handler will catch any exceptions not caught elsewhere and report
+ * them as an error page.
+ *
+ * Each format has its own way of handling exceptions:
+ * - html: exception.default_html, exception.custom_page_html and
+ *   exception.fast_404_html
+ * - json: exception.default_json
+ *
+ * And when the serialization module is installed, all serialization formats are
+ * handled by a single exception subscriber:: serialization.exception.default.
+ *
+ * This exception subscriber runs after all the above (it has a lower priority),
+ * which makes it the last-chance exception handler. It always sends a plain
+ * text response. If it's a displayable error and the error level is configured
+ * to be verbose, then a helpful backtrace is also printed.
+ */
+class FinalExceptionSubscriber implements EventSubscriberInterface {
+  use StringTranslationTrait;
+
+  /**
+   * @var string
+   *
+   * One of the error level constants defined in bootstrap.inc.
+   */
+  protected $errorLevel;
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * Constructs a new FinalExceptionSubscriber.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory) {
+    $this->configFactory = $config_factory;
+  }
+
+  /**
+   * Gets the configured error level.
+   *
+   * @return string
+   */
+  protected function getErrorLevel() {
+    if (!isset($this->errorLevel)) {
+      $this->errorLevel = $this->configFactory->get('system.logging')->get('error_level');
+    }
+    return $this->errorLevel;
+  }
+
+  /**
+   * Handles exceptions for this subscriber.
+   *
+   * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+   *   The event to process.
+   */
+  public function onException(GetResponseForExceptionEvent $event) {
+    $exception = $event->getException();
+    $error = Error::decodeException($exception);
+
+    // Display the message if the current error reporting level allows this type
+    // of message to be displayed, and unconditionally in update.php.
+    $message = '';
+    if ($this->isErrorDisplayable($error)) {
+      // If error type is 'User notice' then treat it as debug information
+      // instead of an error message.
+      // @see debug()
+      if ($error['%type'] == 'User notice') {
+        $error['%type'] = 'Debug';
+      }
+
+      $error = $this->simplifyFileInError($error);
+
+      unset($error['backtrace']);
+
+      if (!$this->isErrorLevelVerbose()) {
+        // Without verbose logging, use a simple message.
+
+        // 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.
+        $message = SafeMarkup::format('%type: @message in %function (line %line of %file).', $error);
+      }
+      else {
+        // With verbose logging, we will also include a backtrace.
+
+        $backtrace_exception = $exception;
+        while ($backtrace_exception->getPrevious()) {
+          $backtrace_exception = $backtrace_exception->getPrevious();
+        }
+        $backtrace = $backtrace_exception->getTrace();
+        // First trace is the error itself, already contained in the message.
+        // While the second trace is the error source and also contained in the
+        // message, the message doesn't contain argument values, so we output it
+        // once more in the backtrace.
+        array_shift($backtrace);
+
+        // Generate a backtrace containing only scalar argument values.
+        $error['@backtrace'] = Error::formatBacktrace($backtrace);
+        $message = SafeMarkup::format('%type: @message in %function (line %line of %file). <pre class="backtrace">@backtrace</pre>', $error);
+      }
+    }
+
+    $content = $this->t('The website encountered an unexpected error. Please try again later.');
+    $content .= $message ? '</br></br>' . $message : '';
+    $response = new Response($content, 500, ['Content-Type' => 'text/plain']);
+
+    if ($exception instanceof HttpExceptionInterface) {
+      $response->setStatusCode($exception->getStatusCode());
+      $response->headers->add($exception->getHeaders());
+    }
+    else {
+      $response->setStatusCode(Response::HTTP_INTERNAL_SERVER_ERROR, '500 Service unavailable (with message)');
+    }
+
+    $event->setResponse($response);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    // Run as the final (very late) KernelEvents::EXCEPTION subscriber.
+    $events[KernelEvents::EXCEPTION][] = ['onException', -256];
+    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);
+    }
+    return $error;
+  }
+
+}