Fatal error: Uncaught Error: Class "Drupal\Core\EventSubscriber\DefaultExceptionHtmlSubscriber" not found in /home/www/drupal8/test/core/lib/Drupal/Core/EventSubscriber/CustomPageExceptionHtmlSubscriber.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/EventSubscriber/CustomPageExceptionHtmlSubscriber.php on line 20