Ajax: Execute JSONP error script responses

Ajax: Execute JSONP error script responses

Issue gh-4379 was meant to be a bug fix but the JSONP case is a bit special:
under the hood it's a script but it simulates JSON responses in an environment
without a CORS setup and sending JSON payloads on error responses is quite
typical there.

This commit makes JSONP error responses still execute the payload. The regular
script error responses continue to be skipped.

Fixes gh-4771
Closes gh-4773