streamlink/ffmpeg output is not logged

[mlindgren]

I'm running twitch-recorder.py as a background job to archive streams. Unfortunately, I've been having a lot of issues lately with the output being truncated, resulting in a broken MP4 file. I've been able to recover the files using https://github.com/anthwlock/untrunc but I still end up losing part of the stream - sometimes a significant part of it.

Unfortunately, the script doesn't log the output of streamlink or ffmpeg, so it's hard to know what's causing these errors.

It would be nice to have the subprocess output captured and included in the log file, at least at debug level.