title: Custom error message from post_save signal
body: <p>Ok so Im saving a CSV document to a Machine object. I want to notifiy the user that the Machine does not exist if it does not exist and exit the save. I first read the CSV file to see what machine the document is for by reading a serial number row.</p>
  
  <p>In my signals:</p>
  
  <pre><code>@receiver(post_save, sender=CSVDocument)
  def read_file(sender,instance, signal, created, **kwargs):
      ...
      machine = Machine.objects.get(serial_number=sn)
      if not machine:
          customNotificationMessage in admin tempalte
  </code></pre>
  
  <p>I do not want to raise an exception at this stage, rather tell the user there is no such machine, so first create the machine.</p>
  
  <p>Currently if I take out the try block to lookupt the machine it raises the exception:</p>
  
  <pre><code>Machine matching query does not exist. Lookup parameters were {'serial_number': 'NEC03610154'}
  </code></pre>
  
  <p>I would actually just want this execption as part of the messages after the file is saved.</p>