The error messages were a bit difficult for users to understand due to a
combination of including useless technical information and lacking
userful user information. The error messages are more verbose now and
give a better indication of what went wrong.
Refactored the ffprobe stream-selection logic to handle rejected files
better:
* Streams tagged as a non-default disposition are not considered
* If a file has any video stream, the audio stream will be ignored
This should prevent videos from being misreported as invalid audio
codecs, etc.
calzoneman