Show separate error for when user tries to cancel letter

that is already cancelled vs when it is too late to
cancel letter vs when we don't know what's the cause
of failure.

This is so we could show useful error messages to the users
and also for better debugging.

tests/app/service/test_rest.py

@@ -3445,6 +3445,7 @@ def test_cancel_notification_for_service_raises_invalid_request_when_letter_is_i
     notification_status,
 ):
     sample_letter_notification.status = notification_status
+    sample_letter_notification.created_at = datetime.now()
 
     response = admin_request.post(
         'service.cancel_notification_for_service',
@@ -3452,7 +3453,13 @@ def test_cancel_notification_for_service_raises_invalid_request_when_letter_is_i
         notification_id=sample_letter_notification.id,
         _expected_status=400
     )
-    assert response['message'] == 'It’s too late to cancel this letter. Printing started today at 5.30pm'
+    if notification_status == 'cancelled':
+        assert response['message'] == 'This letter has already been cancelled.'
+    else:
+        assert response['message'] == (
+            f"We could not cancel this letter. "
+            f"Letter status: {notification_status}, created_at: 2018-07-07 12:00:00"
+        )
     assert response['result'] == 'error'