ffmpeg_cmd and ffmpeg_cmd_async should raise exception if something goes wrong

[balintlaczko]

...instead of misleadingly printing 'Progress 100%'. The exception should print stderr.

[balintlaczko]

Now they do, but I accidentally introduced a regression of not showing progress. FFmpeg does not separate warnings or notes from errors, instead just logs everything into STDERR. And -loglevel error is not a fix (exhibit A...) because it will remove the frame=... progress messages too, basically everything. So have to figure out a way to be able to show progress, but at the same time reliably detect error messages (and distinguish them from warnings).

[balintlaczko]

Manual cropping should also be "gracefully" (automatically) disabled in Colab.

[balintlaczko]

ffmpeg_cmd now raises its FFmpegerror correctly, and can also report progress. Retired the asynchronous version, since the implementation would cost too much time, while there is not much need for this to be async.