title: Custom error message in Django admin actions
body: <p>I've written custom admin actions that basically do <code>QuerySet.update()</code> for certain fields in the model. There are times when these actions shouldn't be allowed to complete -- instead, they should display an error and not do anything. I've tried <code>message_user</code>, but that displays a green checkmark, whereas I'd like it to display the Django admin error message. </p>
  
  <p>A solution I've found online is to use a <code>ModelForm</code>, but I don't think that applies in this case, as here everything happens on the admin change list page.</p>