Attach more information to the FFMpegConvertError errror object

Currently `FFMpegConvertError` exception object doesn't contain a lot of information about the actual error. This makes debugging more time consuming.

I've modified exception to include the full command string of the spawned ffmpeg binary and the output of the ffmpeg command.

Before this change:

```bash
converter.ffmpeg.FFMpegConvertError: Unknown ffmpeg error
```

After this change:

```bash
converter.ffmpeg.FFMpegConvertError: <FFMpegConvertError error="Unknown encoder 'libx264'", cmd="/usr/bin/ffmpeg -i mafile.mov -acodec libmp3lame -ac 2 -ar 11025 -vcodec libx264 -r 15 -s 720x400 -aspect 720:400 -flags +loop -cmp +chroma -partitions +parti4x4+partp8x8+partb8x8 -subq 5 -trellis 1 -refs 1 -coder 0 -me_range 16 -g 300 -keyint_min 25 -sc_threshold 40 -i_qfactor 0.71 -rc_eq 'blurCplx^(1-qComp)' -qcomp 0.6 -qmin 10 -qmax 51 -qdiff 4 -level 30 -f matroska -y out.ogg">
```