EXIT_IF_FAIL was meant to replace

if (!(A)) {
    RING_ERR(__VA_ARGS__);
    loop_.exit();
}

but it is not very readable. Also, loop_.exit() throws an exception
where setup functions calling EXIT_IF_FAIL could simply return
false. Throwing an exception has a higher overhead and leads to less
readable debug output (+ throwing exceptions should never be the
standard way to exit the normal control flow).

This patch
- removes the EXIT_IF_FAIL macro
- replaces code from audio_rtp_session and video_receive_thread which
  was using this macro
- Adds stop() call at the end of ThreadLoop::mainloop. At this point
  the thread loop is obviously not executing anymore.

Change-Id: Id1ed8eb914e91686a63775eba3ad70f8b6143ae5
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>