HTTP HOOK: return detail error in http response

[thegobot]

Return an error from the application or the SRS itself in headers or status codes. This will help you understand why the stream failed to start, either from the application code or the internal state of the SRS.

Example

on_publish "https://example.com/hook/srs/publish";

if stream busy: return status code 100 Continue OR in Headers (the end user will understand that the old stream needs to be stopped)
if error drom application: return response code (proxy?) OR in Headers "status text"

For now, this can only be applied to WHIP

[winlinvip]

That makes sense; it's worth improving the error message to clearly indicate the root cause.