Ensure an error gets logged when a delivery method fails (#3144)

* Ensure an error gets logged when a delivery method fails

- Refactor existing code to log a custom error to the database
- Limit error notifications to UI

* Adjust unit test

* Clear existing notifications before run

* Add some debug to work out what is going on

* Accommodate extra notification

InvenTree/part/test_part.py

@@ -616,6 +616,8 @@ class BaseNotificationIntegrationTest(InvenTreeTestCase):
         # reload notification methods
         storage.collect(run_class)
 
+        NotificationEntry.objects.all().delete()
+
         # There should be no notification runs
         self.assertEqual(NotificationEntry.objects.all().count(), 0)
 
@@ -630,8 +632,8 @@ class BaseNotificationIntegrationTest(InvenTreeTestCase):
         self.part.set_starred(self.user, True)
         self.part.save()
 
-        # There should be 1 notification
-        self.assertEqual(NotificationEntry.objects.all().count(), 1)
+        # There should be 1 (or 2) notifications - in some cases an error is generated, which creates a subsequent notification
+        self.assertIn(NotificationEntry.objects.all().count(), [1, 2])
 
 
 class PartNotificationTest(BaseNotificationIntegrationTest):