class CustomPageExceptionHtmlSubscriber

class CustomPageExceptionHtmlSubscriber extends DefaultExceptionHtmlSubscriber {
    
    /**
     * The configuration factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The access manager.
     *
     * @var \Drupal\Core\Access\AccessManagerInterface
     */
    protected $accessManager;
    
    /**
     * Constructs a new CustomPageExceptionHtmlSubscriber.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The configuration factory.
     * @param \Symfony\Component\HttpKernel\HttpKernelInterface $http_kernel
     *   The HTTP Kernel service.
     * @param \Psr\Log\LoggerInterface $logger
     *   The logger service.
     * @param \Drupal\Core\Routing\RedirectDestinationInterface $redirect_destination
     *   The redirect destination service.
     * @param \Symfony\Component\Routing\Matcher\UrlMatcherInterface $access_unaware_router
     *   A router implementation which does not check access.
     * @param \Drupal\Core\Access\AccessManagerInterface $access_manager
     *   The access manager.
     */
    public function __construct(ConfigFactoryInterface $config_factory, HttpKernelInterface $http_kernel, LoggerInterface $logger, RedirectDestinationInterface $redirect_destination, UrlMatcherInterface $access_unaware_router, AccessManagerInterface $access_manager) {
        parent::__construct($http_kernel, $logger, $redirect_destination, $access_unaware_router);
        $this->configFactory = $config_factory;
        $this->accessManager = $access_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function getPriority() {
        return -50;
    }
    
    /**
     * {@inheritdoc}
     */
    public function on403(GetResponseForExceptionEvent $event) {
        $custom_403_path = $this->configFactory
            ->get('system.site')
            ->get('page.403');
        if (!empty($custom_403_path)) {
            $this->makeSubrequestToCustomPath($event, $custom_403_path, Response::HTTP_FORBIDDEN);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function on404(GetResponseForExceptionEvent $event) {
        $custom_404_path = $this->configFactory
            ->get('system.site')
            ->get('page.404');
        if (!empty($custom_404_path)) {
            $this->makeSubrequestToCustomPath($event, $custom_404_path, Response::HTTP_NOT_FOUND);
        }
    }
    
    /**
     * Makes a subrequest to retrieve the custom error page.
     *
     * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
     *   The event to process.
     * @param string $custom_path
     *   The custom path to which to make a subrequest for this error message.
     * @param int $status_code
     *   The status code for the error being handled.
     */
    protected function makeSubrequestToCustomPath(GetResponseForExceptionEvent $event, $custom_path, $status_code) {
        $url = Url::fromUserInput($custom_path);
        if ($url->isRouted()) {
            $access_result = $this->accessManager
                ->checkNamedRoute($url->getRouteName(), $url->getRouteParameters(), NULL, TRUE);
            $request = $event->getRequest();
            // Merge the custom path's route's access result's cacheability metadata
            // with the existing one (from the master request), otherwise create it.
            if (!$request->attributes
                ->has(AccessAwareRouterInterface::ACCESS_RESULT)) {
                $request->attributes
                    ->set(AccessAwareRouterInterface::ACCESS_RESULT, $access_result);
            }
            else {
                $existing_access_result = $request->attributes
                    ->get(AccessAwareRouterInterface::ACCESS_RESULT);
                if ($existing_access_result instanceof RefinableCacheableDependencyInterface) {
                    $existing_access_result->addCacheableDependency($access_result);
                }
            }
            // Only perform the subrequest if the custom path is actually accessible.
            if (!$access_result->isAllowed()) {
                return;
            }
        }
        $this->makeSubrequest($event, $custom_path, $status_code);
    }

}