obs-ffmpeg: Add error detection to ffmpeg-mux

ffmpeg-mux does not notice if ffmpeg returns an error from
av_interleaved_write_frame() which means that OBS never knows if there is a
problem in ffmpeg.

This is the biggest issue for cases like srt:// or tcp:// streams that can
regularly fail. Without this change OBS never knows that something went wrong.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f