title: Django: Display a custom error message for admin validation error
body: <p>I'm using Django 1.2.4. I have a model that has a field that needs to be validated. When validation fails, I'd like to display a custom error message to the user. Model editing is done in the admin interface. </p>
  
  <p>This is what I'm doing currently:</p>
  
  <pre><code>def clean_fields(self, exclude=None):
      # do validation
      if problem:
          raise ValidationError({'field_name': "error message"})
  </code></pre>
  
  <p>Unfortunately, all this does is print out a separate validation message on the admin page for each character in the value of <code>field_name</code>.</p>
  
  <p>What is the proper way to signal the error message I want?</p>