Return GraphQL JSON response in case of JSON Input error instead of Internal Server Error

[cvergne]

Fixes #207

---

The purpose of the PR is to return a standard GraphQL JSON error response in case of JSON error in given input (request body).

{
    "errors": [
        {
            "message": "Invalid JSON.",
            "extensions": {
                "reason": "Syntax error"
            }
        }
    ]
}

It adds an Exception Listener which set a JsonResponse in case of thrown GraphQLExceptionInterface (to catch any GraphQL Exception outside of the GraphQL Server context).

Also added the two exception cases of the controller into errors functional tests.

Note

Got an error when running test about DependencyInjection/Configuration.php not compatible with implemented interface, so fixed it but tell me if I should remove it and having it fixed in another PR.

[homersimpsons]

I will let @mistraloz review this as you discussed it.

About the implementation I'm okay with the current status. But I'm feeling that it may be too complex, this adds a lew listener, a new exception (which names conflict with a std one).

I think this is the right way to approach this as the end user will have enough control on it. But maybe a simple return new JsonResponse(..., 422) would have been sufficient.

Thank you for the tests too!

[cvergne]

May we reopen this pull request as it just needs review ? :)