AlertEngine crashes when processing filters containing "not equals" matches

[lunkwill42]

Ridiculously enough, it seems the AlertEngine does not support filters that have "not equals" match expressions in them, and has never done so in its current incarnation (approximately the last 9 years or so).

Adding such a filter to an alert profile may cause the Alert Engine to crash with this traceback logged:

Traceback (most recent call last):
  File "/usr/lib/nav/alertengine.py", line 162, in main
    check_alerts(debug=opttest)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 63, in check_alerts
    handle_new_alerts(new_alerts)
  File "/usr/lib/python2.7/dist-packages/django/db/transaction.py", line 394, in inner
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 186, in handle_new_alerts
    permissions)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 200, in _check_match_and_permission
    if memoized_check_alert(alert, filtergroupcontents, 'match check'):
  File "/usr/lib/python2.7/dist-packages/django/utils/functional.py", line 38, in wrapper
    result = func(*args)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 450, in check_alert_against_filtergroupcontents
    matches = content.filter.check(alert) == content.positive
  File "/usr/lib/python2.7/dist-packages/nav/models/profiles.py", line 852, in check
    expression.get_operator_mapping())
  File "/usr/lib/python2.7/dist-packages/nav/models/profiles.py", line 761, in get_operator_mapping
    return Operator(type=self.operator).get_operator_mapping()
  File "/usr/lib/python2.7/dist-packages/nav/models/profiles.py", line 735, in get_operator_mapping
    return self.OPERATOR_MAPPING[self.type]
KeyError: 5