A BugWatch's last_error_type should be taken from its most recent activity

[Graham Binns]

At the moment, BugWatch.last_error_type is stored in the database. However, this is duplicated data, since we record it in BugWatchActivity.result. We should remove the db field and instead return the result of the most recent BugWatchActivity entry as BugWatch.last_error_type.

Note that if the last BugWatchActivity.result was one of BUG_WATCH_ACTIVITY_SUCCESS_STATUSES (in lp.bugs.interfaces.bugwatch) BugWatch.last_error_type should return None, since there was no error.