Provide a better exception message

features/security/validate_response_types.feature

@@ -28,11 +28,11 @@ Feature: Validate response types
     And I send a "GET" request to "/dummies/1.invalid"
     Then the response status code should be 404
     And the header "Content-Type" should be equal to "application/problem+json; charset=utf-8"
-    And the JSON node "detail" should be equal to "Not Found"
+    And the JSON node "detail" should be equal to 'Format "invalid" is not supported'
 
   Scenario: Requesting an invalid format in the Accept header and in the URL should throw an error
     When I add "Accept" header equal to "text/invalid"
     And I send a "GET" request to "/dummies/1.invalid"
     Then the response status code should be 404
     And the header "Content-Type" should be equal to "application/problem+json; charset=utf-8"
-    And the JSON node "detail" should be equal to "Not Found"
+    And the JSON node "detail" should be equal to 'Format "invalid" is not supported'

src/EventListener/AddFormatListener.php

@@ -58,7 +58,7 @@ public function onKernelRequest(GetResponseEvent $event)
         if (null === $routeFormat = $request->attributes->get('_format') ?: null) {
             $mimeTypes = array_keys($this->mimeTypes);
         } elseif (!isset($this->formats[$routeFormat])) {
-            throw new NotFoundHttpException('Not Found');
+            throw new NotFoundHttpException(sprintf('Format "%s" is not supported', $routeFormat));
         } else {
             $mimeTypes = Request::getMimeTypes($routeFormat);
         }

tests/EventListener/AddFormatListenerTest.php

@@ -171,7 +171,7 @@ public function testAcceptHeaderTakePrecedenceOverRequestFormat()
 
     /**
      * @expectedException \Symfony\Component\HttpKernel\Exception\NotFoundHttpException
-     * @expectedExceptionMessage Not Found
+     * @expectedExceptionMessage Format "invalid" is not supported
      */
     public function testInvalidRouteFormat()
     {