HTTP error code is not correctly fetched from exception code

The HTTP code of the error page is not correctly fetched from the exception code.
This PR starts with a failing test.

See #17

Author: moufmouf

Tests/Fixtures/Controller/TestGraphqlController.php

@@ -61,8 +61,8 @@ public function contacts(): ArrayResult
      * @Query()
      * @return string
      */
-    public function triggerError(): string
+    public function triggerError(int $code = 500): string
     {
-        throw new MyException('Boom');
+        throw new MyException('Boom', $code);
     }
 }

Tests/FunctionalTest.php

@@ -113,5 +113,13 @@ public function testErrors()
 
         $this->assertSame(500, $response->getStatusCode());
 
+        $request = Request::create('/graphql', 'GET', ['query' => '
+        { 
+          triggerError(code: 404)
+        }']);
+
+        $response = $kernel->handle($request);
+
+        $this->assertSame(404, $response->getStatusCode(), $response->getContent());
     }
 }