title: Django Forms: if not valid, show form with error message
body: <p>In Django forms, it can check whether the form is valid:</p>
  <pre><code>if form.is_valid(): 
      return HttpResponseRedirect('/thanks/')
  </code></pre>
  <p>But I'm missing what to do if it isn't valid? How do I return the form with the error messages? I'm not seeing the &quot;else&quot; in any of the examples.</p>