Unify validation exception formatting.

[taylorotwell]

This unifies all validation error formatting into the exception handler
and out of Form Request. “Don’t Flash” is typically a global concept
that always applies to a list of fields for all pages in the
applications, so seemed to make sense to specify in one place instead
of each form.

Total control of the Form Request error response can still be gained by
overriding failedValidation and throwing a HttpResponseException with
whatever response you want.

This would mean in Laravel 5.5 all validation JSON responses would be customized in a single location within your application's exception handler:

    /**
     * Convert a validation exception into a JSON response.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Illuminate\Validation\ValidationException  $exception
     * @return \Illuminate\Http\Response
     */
    protected function invalidJson($request, ValidationException $exception)
    {
        return response()->json([
            'message' => $exception->getMessage(),
            'errors' => $exception->errors(),
        ], 422);
    }