[11.x] Provide an error message for PostTooLargeException

[patrickomeara]

The ValidatePostSize middleware ensures the POST is under the allowed size set in php.ini.

However the exception message is empty

{
   "message": ""
}

This PR adds a message.

[rodrigopedra]

Not all the Exceptions extending HttpException will have a message (I am saying not all, instead of none as being too busy right now to test all of them).

For example:

Route::get('test', function (Request $request) {
    // ensure result is a JSON
    $request->headers->set('Accept', 'application/json');

    return throw new NotFoundHttpException();
});

Will give you the same response:

{
    "message": ""
}

Also, in case this is desirable, please use the default HTTP message for this exception: Content Too Large (reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/413 )

---

EDIT:

Forgot to mention, that the response's status code (413) should suffice for client applications to know the error.

Although, the linked MDN article has a nice suggestion on informing the client of the actual accepted size limit. Which could be useful.