title: Django 1.5.4 error_messages in ModelForm
body: <p>I have a very simple ModelForm subclass:</p>
  
  <pre><code>class UserPrefsForm(forms.ModelForm):
      class Meta:
          model = User
          fields = ['first_name','last_name','username']
  </code></pre>
  
  <p>When I try to save the form in my view (only the post function here):</p>
  
  <pre><code>def post(self, request):
      userform =  UserPrefsForm(request.POST)
      context = self.get_context_data()
      if userform.is_valid():
          userform.save()
      else:
          context['errors'] = True
      return render(request,self.template_name,context)
  </code></pre>
  
  <p>I got this error:</p>
  
  <pre><code>AttributeError at /prefs
  'super' object has no attribute 'error_messages'
  Traceback:
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
    115.                         response = callback(request, *callback_args, **callback_kwargs)
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/views/generic/base.py" in view
    68.             return self.dispatch(request, *args, **kwargs)
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/braces/views.py" in dispatch
    107.             request, *args, **kwargs)
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/views/generic/base.py" in dispatch
    86.         return handler(request, *args, **kwargs)
  File "/home/tonjo/prj/tuned_prj/tuned_django/graph/views.py" in post
    147.         if userform.is_valid():
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/forms/forms.py" in is_valid
    126.         return self.is_bound and not bool(self.errors)
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/forms/forms.py" in _get_errors
    117.             self.full_clean()
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/forms/forms.py" in full_clean
    274.         self._post_clean()
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/forms/models.py" in _post_clean
    344.             self.validate_unique()
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/forms/models.py" in validate_unique
    353.             self.instance.validate_unique(exclude=exclude)
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/db/models/base.py" in validate_unique
    731.         errors = self._perform_unique_checks(unique_checks)
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/db/models/base.py" in _perform_unique_checks
    831.                 errors.setdefault(key, []).append(self.unique_error_message(model_class, unique_check))
  File "/home/tonjo/venv/tuned/local/lib/python2.7/site-packages/django/db/models/base.py" in unique_error_message
    882.             return field.error_messages['unique'] % {
  File "/home/tonjo/venv/tuned/src/neo4django/neo4django/utils.py" in __getattr__
    303.         return getattr(target, name)
  </code></pre>
  
  <p>I understand is the missing <code>error_messages</code>... Am I forced to define them?</p>