Improve ffmpeg exit logging
I'm setting up a new camera on a new system, and I got this error message:

`[7/2/2020, 9:34:48 PM] [Camera-ffmpeg] [FFMPEG] ffmpeg exited with code: 1 and signal: null (error)` 

That is despite having debug enabled (`cameras[].videoConfig.debug: true`) . Clearly, there are not enough details to help me troubleshoot the issue.

I wish I could more easily see FFmpeg error output. Maybe include a path to a log file?