[TwigBundle] fixed a PHP warning (closes #4397)

src/Symfony/Bundle/TwigBundle/Controller/ExceptionController.php

@@ -46,7 +46,7 @@ class ExceptionController extends ContainerAware
             $this->findTemplate($templating, $format, $code, $this->container->get('kernel')->isDebug()),
             array(
                 'status_code'    => $code,
-                'status_text'    => Response::$statusTexts[$code],
+                'status_text'    => isset(Response::$statusTexts[$code]) ? Response::$statusTexts[$code] : '',
                 'exception'      => $exception,
                 'logger'         => $logger,
                 'currentContent' => $currentContent,