Machine-readable errors

[fstirlitz]

There should be an easy way to automatically gather error information when youtube-dl fails. Specifically, I would like to easily detect whether youtube-dl failed to recognise a given URL at all, whether there was a network error, etc.

Some suggestions:

• Use exit codes other than 0 or 1.
• youtube-dl -j should report errors on stdout in a JSON object, containing a human-readable message and a machine-readable error code. (To compare, livestreamer does something like that, but it's rather half-hearted: only the error message is wrapped inside JSON.)