Throw an exception whenever we updated a notification to technical failure.

If this is happening we want to know about it.

app/exceptions.py

@@ -1,3 +1,8 @@
 class DVLAException(Exception):
     def __init__(self, message):
         self.message = message
+
+
+class NotificationTechnicalFailureException(Exception):
+    def __init__(self, message):
+        self.message = message