log errors adding users already in db

avoids removal of system users preventing Hub startup

jupyterhub/app.py

@@ -855,7 +855,13 @@ def init_users(self):
         # but changes to the whitelist can occur in the database,
         # and persist across sessions.
         for user in db.query(orm.User):
-            yield gen.maybe_future(self.authenticator.add_user(user))
+            try:
+                yield gen.maybe_future(self.authenticator.add_user(user))
+            except Exception:
+                # TODO: known cause of this is system users that have been removed.
+                # Should we be removing from the db in this case?
+                # API abtractions makes it hard to know that this is what happened at this level.
+                self.log.exception("Error adding user %r already in db", user.name)
         db.commit() # can add_user touch the db?
 
         # The whitelist set and the users in the db are now the same.