fix for files matched to tracks that disappear from musicbrainz

[canidae]

musicbrainz occasionally remove albums (ie. merging two albums makes one disappear along with the unique musicbrainz ids), which leaves us with a sorted file that got no valid link to a track in the musicbrainz database.
most likely this is just to remove the musicbrainz ids (only from the database, not the file) and do a metadata search which hopefully should come up with a valid match.