replaygain: More descriptive backend errors

This avoids a bare `except:` to catch specific errors and give better messages
when they fail. Should provide more insight to this trouble:
https://groups.google.com/d/msg/beets-users/6d-TOg0SuPY/iL39tmo3rqUJ

Also related to #874.

Feedback from @yevgenybezman or others more familiar with the backend would be
welcome.