A few changes to the badfiles plugin

[karpinski]

Adding a verbosity option per request from #1654.

Process now continues to other files instead of stopping after a checker error (#2430).

The checker errors are logged only once for each file extension by default and for each file separately in the verbose mode. Let me know if this is fine or if you'd rather have just one of those.

[sampsyo]

Thanks for getting this started!

Is there any chance I could convince you to break this out into two separate PRs? The discussion here might get a little bit messy otherwise. It's possible we can resolve everything easily, of course, but if not, it might be nice to have two separate discussions.

For the verbosity mode: beets already has a global verbosity mode, which affects the logging level. Can we just use that judiciously instead of introducing a new flag? That would just mean sending everything to debug output except for the important messages.

For the new exception handling, throwing a new exception is fine. But it seems like an easier implementation here would just log a message and continue—that would be a bit simpler than building up a list of missing checkers. I know that will make the output more verbose, but I also think it will be slightly less surprising to use. Does that make sense?

[karpinski]

Thanks for the feedback, @sampsyo! I've opened new PRs for those separate changes.