title: Django ModelForm validation and error messages
body: <p>Sorry if this is a basic question: I am having trouble doing form validation with ModelForms in Django.</p>
  
  <p>The pattern I'm using is</p>
  
  <pre><code>def View(request):
      if request.method == 'POST':
  
          form = AddPageForm(request.POST)
  
          if form.is_valid:
              instance = form.save()  
              ...
  
          else:
              HttpResponse("Error")
  </code></pre>
  
  <p>This works fine if the form validates (the <code>if</code>... branch is followed.) When the form doesn't validate, I get a standard Django form validation error page; the <code>else</code>... branch is ignored.</p>
  
  <p>Obviously, there must be something wrong/missing but I can't work out what from the official Django documentation. Any guidance would be appreciated.</p>