public function onResponse(Event $event)
 {
     $response = $event->getResponse();
     $code = $response->getCode();
     if (false === $event->getDebug()) {
         if ($code == 404) {
             $response->setBody('Page not found.');
         }
         if ($code == 500) {
             $response->setBody('Internal error.');
         }
     }
     $response->send();
 }