title: adding errors to Django form errors.__all__
body: <p>How do I add errors to the top of a form after I cleaned the data? I have an object that needs to make a REST call to an external app (google maps) as a pre-save condition, and this can fail, which means I need my users to correct the data in the form. So I clean the data and then try to save and add to the form errors if the save doesn't work:</p>
  
  <pre><code>if request.method == "POST":
  #clean form data
      try:
          profile.save()
          return HttpResponseRedirect(reverse("some_page", args=[some.args]))
      except ValueError:
          our_form.errors.__all__ = [u"error message goes here"]
  return render_to_response(template_name, {"ourform": our_form,}, 
         context_instance=RequestContext(request))
  </code></pre>
  
  <p>This failed to return the error text in my unit-tests (which were looking for it in <code>{{form.non_field_errors}}</code>), and then when I run it through the debugger, the errors had not been added to the forms error dict when they reach the <code>render_to_response</code> line, nor anywhere else in the our_form tree. Why didn't this work? How am I supposed to add errors to the top of a form after it's been cleaned?</p>