Are you using a response node in your workflow? If so, you should be able to setup the event handlers on the Workflow query to show the success message if the response comes back the way you require.
I think you can return anything you want in the body here because you can check the Status Code: 200 in the App's success event. Also, I believe you can create a response block that is a global error handler and send back an error status message and code (but I haven't tried that out yet, just had a thought).