checkwatches is fragile when non-existent bugtrackers are specified at the command line

[Graham Binns]

If you use `checkwatches.py -t $bugtracker` to specify a bugtracker to update, checkwatches will error messily:

Traceback (most recent call last):
  File "./checkwatches.py", line 39, in ?
    script.lock_and_run()
  File "/srv/staging.launchpad.net/staging/launchpad/lib/canonical/launchpad/scripts/base.py", line 264, in lock_and_run
    implicit_begin=implicit_begin, isolation=isolation)
  File "/srv/staging.launchpad.net/staging/launchpad/lib/canonical/launchpad/scripts/base.py", line 224, in run
    self.main()
  File "./checkwatches.py", line 31, in main
    updater.updateBugTrackers(self.options.bug_trackers)
  File "/srv/staging.launchpad.net/staging/launchpad/cronscripts/../lib/canonical/launchpad/scripts/checkwatches.py", line 189, in updateBugTrackers
    bug_tracker_url = bug_tracker.baseurl
AttributeError: 'NoneType' object has no attribute 'baseurl'

This should at least be an OOPS, if not just a log message (since it's being run manually in such a case).

[Graham Binns (gmb)]

Rats. This should have been a malone bug; apologies.