Submitting a threshold rule without a target crashes with a KeyError

[lunkwill42]

In NAV 4.8, when attempting to submit a new threshold rule in the Treshold tool, if the target field is left empty, the page crashes with a KeyError instead of simply reporting an invalid form submission:

Traceback:
File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py" in get_response
  111.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/source/python/nav/web/threshold/views.py" in add_threshold
  56.         if form.is_valid():
File "/usr/local/lib/python2.7/dist-packages/django/forms/forms.py" in is_valid
  162.         return self.is_bound and not bool(self.errors)
File "/usr/local/lib/python2.7/dist-packages/django/forms/forms.py" in errors
  154.             self.full_clean()
File "/usr/local/lib/python2.7/dist-packages/django/forms/forms.py" in full_clean
  353.         self._clean_fields()
File "/usr/local/lib/python2.7/dist-packages/django/forms/forms.py" in _clean_fields
  371.                     value = getattr(self, 'clean_%s' % name)()
File "/source/python/nav/web/threshold/forms.py" in clean_alert
  100.         validate_expression(alert, self)
File "/source/python/nav/web/threshold/forms.py" in validate_expression
  121.     target = form.cleaned_data['target']

Exception Type: KeyError at /threshold/add
Exception Value: 'target'