actions that complete and return a result but are application errors return BadGateway instead of OK

The controller rewrites the activation record into the following fields:
- success: true|false
- statusMessage: "success"|"application error"|"developer error"|"internal error"
- result: {...}|{"error": ...}

The invariants are that `sucess=true` _iff_ `statusMessage="sucess"` _iff_ `result.error == undefined`.

The actual HTTP response code to the POST (action invoke) request is 200 for success **and** application errors, 502 for action errors, and 500 for internal errors.

Currently the implementation return 502 for application error.

Need to add a test and fix.