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.
Only network streams are checked to prevent impacting potential
transient errors in recordings.
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