Improve error message when assigned to MR with unresolved threads

Currently, the message posted by marge on when attempting to merge an MR with outstanding threads is only:

I couldn't merge this branch: Merge request was rejected by GitLab: 'Branch cannot be merged'

and the log message is:

| 2021-07-08 11:01:26,671 WARNING I couldn't merge this branch: Merge request was rejected by GitLab: 'Branch cannot be merged'

This could be improved by extending

def ensure_mergeable_mr(self, merge_request):

by retrieving all notes in the mr, and checking that all notes that are resolvable are also resolved.

I think it might be even simpler than that. In the GitLab MR API there's a JSON key blocking_discussions_resolved which we can check.

OK, PR up and ready for review...