Encoder must properly parse error messages in case of failure

[zeh]

Currently, when the encoder fails (that is, FFmpeg finishes with a non-zero termination code), the progress bar changes to a "Failed" state, with no indication on what happened and why.

We need to properly parse that error message, and expose it to the user.

When diagnosing memory issues (see #8), @viv@snoot.tube had a good example case of an error state that didn't report its error message (https://snoot.tube/@viv/110858063043939012):

...
[FFMPEG][stderr] frame= 8 fps=0.9 q=0.0 size= 0kB time=00:00:01.20 bitrate= 0.3kbits/s speed=0.137x
[FFMPEG][stderr] x264 [error]: malloc of size 34241728 failed
[FFMPEG][stderr] Error submitting video frame to the encoder
[FFMPEG][stderr] [aac @ 0x3214430] Qavg: 181.749
[FFMPEG][stderr] [aac @ 0x3214430] 2 frames left in the queue on closing
[FFMPEG][stderr] Conversion failed!
[FFMPEG][stderr] Aborted()

We need to try and parse lines that are error-specific, and add them to an error message field.

Requirements

• Parse error messages from encoder as they come
• Expose error messages to the user through the progress bar