Handle mutagen errors better

This adds a new menu entry to store broken files, rather than leaving them in unmatched files.  

It also, at long last, silences all the mutagen log spam which happens each time Picard cannot load metadata because the file is broken.  It seems not a Picard bug for a broken file to not be readable; that would seem to be the *expected* behaviour, and thus not something that should be spamming the log with tracebacks.

Last, this adds a new plugin register point, to allow for plugins to target only the Broken Files menu item's plugin menu.

Note: I would have liked to create clusters in there, to split the broken files out into pseudo-clusters depending on what was broken (based on the exception thrown), but I couldn't get those clusters to go under the Broken Files item; they always ended up showing under the Clusters item.  :(