The FormRequest should throw an AuthorizationException which the framework will convert to a Response with a 403 status, and respecting the Accept header, i.e. if you want JSON it will return JSON.
You can customise how an AuthorizationException is handled in the App\Exceptions\Handler class (pre-L11) or you can use the withExceptions method when bootstrapping the app in bootstrap/app.php to determine how it should be rendered by your application.
@tykus Sorry, my bad, I didn't explain myself well.
It does return JSON, but it returns the Exception so my real question is, without messing with app.php in bootstrap, is there a way to convert this Exception to error message JSON object?
@Mrtvac yes, as I explained above, your message is there. The trace is included whenever debug mode is enabled. If you change the APP_DEBUG in your environment file to false; it will only have the message key