lucifer: Hi! I'm looking at Sentry tickets, and this one makes me think it is possible to set up the lastfm importer with an empty username, which results in a 400 error:
https://sentry.metabrainz.org/organizations/metabrainz/issues/798
Note the `&user=&from=...` empty username in the request URL. I didn't think a user could set it up with an empty username from the front-end, but in any case I think that brings up two issues:
1. the /music-service/XXX/connect/ endpoint should be more constrained and ensure the username is not empty? Maybe even check if the user exists?
2. We should surface some errors to the front-end, possibly with flash messages that are set up, to let users know when we consistently haven't been able to import their listens