Report bootstrap failures/stalls more informatively
When we don't manage to get a good directory, etc., we tned to report bland messages like "Can't bootstrap a Tor directory". This isn't very helpful. We should do better.
This is nontrivial. As @nickm writes !587 (comment 2813202):
Each possible [error] variant can be caused by a bunch of different causes, and we don't know which are important to notice and track.