Pass more FFmpeg error details back to the caller

[farski]

[farski]

@cavis Once #187 is merged, it potentially becomes a lot easier to cherry pick what result/error info comes out of the task and ends up in callbacks. Was there anything in particular we wanted to improve with this?

[cavis]

I don't see a way to get nicely formatted errors out of it, so to start maybe we just capture stdout and send that back to the caller to interpret? Even being able to see them in the rails console would be helpful.

For example, this bad mjpeg file:

# capture this stderr
ffmpeg -i 303bb935-d159-44c0-acdf-788cb19fffdc.mp3 out.mp3

# or use the ffreport thing?
FFREPORT=file=ffreport.log:level=16 ffmpeg -i 303bb935-d159-44c0-acdf-788cb19fffdc.mp3 out.mp3

Does seem to be some structure to ffmpeg errors... but i wouldn't want to rely on parsing that upfront.