Display cancelled letters show as failed

In the long term, we don't want to show cancelled letters. But for now,
this changes cancelled letters to display in the same way that letters
with a status of permanent-failure, since we are currently giving
letters that we want to cancel the status of permanent failure.

app/__init__.py

@@ -373,6 +373,7 @@ def format_notification_status(status, template_type):
             'pending-virus-check': 'Pending virus check',
             'virus-scan-failed': 'Virus detected',
             'returned-letter': 'Delivered',
+            'cancelled': 'Cancelled,'
         }
     }[template_type].get(status, status)
 
@@ -398,6 +399,7 @@ def format_notification_status_as_field_status(status, notification_type):
             'pending-virus-check': None,
             'virus-scan-failed': 'error',
             'returned-letter': None,
+            'cancelled': 'error',
         }
     }.get(
         notification_type,