rtsp-server: Add support for adjusting request response on pipeline errors
The idea is to give the application the possibility to adjust the error code when responding to a request. For that purpose the pipeline errors are stored in a list in rtsp-media, so that they can be checked by the application. That is done by overriding a new virtual method in rtsp-client, called adjust_error_code which will be executed before sending the response.
Fixes #1294 (closed)