caa-fetcher: Allow pasting musicbrainz URIs for MBIDs

This alters the input parsing so you can simply paste any string
containing a substring that looks like an MBID.  The first such
substring will be used.  This makes it easier to browse to a release
on Musicbrainz and just copy+paste the URI into caa-fetcher.

And improve the error messages if you paste some other kind of MBID.