Attach more information to the FFMpegConvertError errror object and update
affected code.
Remove invalid plug sign (+) from the chroma argument which is passed to the
cmp option.
Merge pull request #12 from Kami/improve_exceptions

Attach more information to the FFMpegConvertError errror object
Merge pull request #13 from Kami/update_ffmpeg_options

Update ffmpeg options