title: Django error message displayed for unique fields
body: <p>I would like to change the default error message when duplicate entries try to save when they should be unique i.e. <code>unique=True</code>. Much like this:</p>
  
  <pre><code>email = models.EmailField(unique=True, error_messages={'unique':"This email has already been registered."})
  </code></pre>
  
  <p>But, <code>unique</code> in the above case was a guess, and doesn't work. Neither can I find out what the name of the error actually is. Does anyone know the correct name?</p>
  
  <p>Note, this validation is model level, not form validation.</p>
  
  <p>EDIT:
  A bit more info, at the moment the current error message is displayed by <code>form.errors</code>:</p>
  
  <pre><code>[model_name] with this [field_label] already exists
  </code></pre>
  
  <p>Which isn't very user friendly, so I wanted to override it...</p>