BugTrackerAuthenticationError updating bugzilla's bugs

[Ursula Junque]

As seen on OOPS-1866CCW1104 and OOPS-1866CCW1301:

   Traceback (most recent call last):
  Module lp.bugs.scripts.checkwatches.bugwatchupdater, line 87, in updateBugWatch
    self.pushBugComments()
  Module lp.bugs.scripts.checkwatches.base, line 266, in wrapper
    return func(self, *args, **kwargs)
  Module lp.bugs.scripts.checkwatches.bugwatchupdater, line 241, in pushBugComments
    message_rfc822msgid))
  Module lp.bugs.externalbugtracker.isolation, line 63, in wrapper
    return func(*args, **kwargs)
  Module lp.bugs.externalbugtracker.bugzilla, line 516, in decorator
    self._authenticate()
  Module lp.bugs.externalbugtracker.isolation, line 63, in wrapper
    return func(*args, **kwargs)
  Module lp.bugs.externalbugtracker.bugzilla, line 581, in _authenticate
    self.xmlrpc_proxy.User.login(self.credentials)
  Module lp.bugs.externalbugtracker.bugzilla, line 569, in credentials
    self.baseurl, "No credentials found.")
BugTrackerAuthenticationError: https://bugzilla.novell.com: No credentials found.