BugzillaAPI._storeBugs() doesn't handle bugs not having an alias field

Steps to reproduce:
Execute the following in bin/harness:
>>> from lp.bugs.externalbugtracker.bugzilla import Bugzilla
>>> b = Bugzilla('https://wwws.clamav.net/bugzilla')
>>> b = b.getExternalBugTrackerToUse()
>>> b.initializeRemoteBugDB(['2296'])

What happens:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/graham/launchpad/lp-branches/make-mailnotification-care-bug-656759/lib/lp/bugs/externalbugtracker/isolation.py", line 63, in wrapper
    return func(*args, **kwargs)
  File "/home/graham/launchpad/lp-branches/make-mailnotification-care-bug-656759/lib/lp/bugs/externalbugtracker/bugzilla.py", line 650, in initializeRemoteBugDB
    self._storeBugs(remote_bugs)
  File "/home/graham/launchpad/lp-branches/make-mailnotification-care-bug-656759/lib/lp/bugs/externalbugtracker/bugzilla.py", line 578, in _storeBugs
    if remote_bug['alias'] != '':
KeyError: 'alias'

What should happen:
_storeBugs() should handle the lack of an alias field appropriately (and if it's indicative of data being wonky, should handle *that* appropriately) instead of letting the KeyError bubble up.