title: Return error message after saving a model with custom save() method on Django Admin
body: <p>I've two models. One is a <code>Subscriber</code> and at save time I'm assigning a pool based on the number of subscribers already assigned to that pool:</p>
  
  <pre><code>class Subscriber(models.Model):
      pool = models.ForeignKey(Pool)
      interface = models.CharField(max_length=30)
  
  
  class Pool(models.Model):
      name = models.CharField(max_length=50)
  </code></pre>
  
  <p>Let's say that only 4 <code>Subscribers</code> can be assigned to a pool. That's why I'm overriding Subscriber's <code>save()</code> method:</p>
  
  <pre><code>def save(self, *args, **kwargs):
      if not self.pk:
          #Look for a free Pool among the available ones
          for pool in Pool.objects.all():
              if pool.subscriber_set.count() &lt; 4:
                  self.pool = pool
                  print "Assigned pool: %s" % pool.name
                  super(Subscriber, self).save(*args, **kwargs)
  </code></pre>
  
  <p>It's working great until I run out of pools (all of them have 4 users assigned). How should I handle this from Django admin? Ideally I'd like to show some error message to the user so he can create more pools. </p>
  
  <p>I would rather not move the pool assignation code to the form's <code>clean()</code> method because probably users will also be created from a different interface, not the admin GUI. </p>
  
  <p>Any ideas?</p>
  
  <p>Thanks a lot! </p>