Raise error if multiple broadcasts found for reference

Because the `<reference>` field of a `cancel` message can contain an
arbitrary number of items it’s possible for it to reference more than
one current alert.

In this case it is ambiguous which alert should be cancelled, so we
should raise a custom error.

This will help people know that they have to manually go into Notify and
figure out which alert(s) to cancel there.

app/v2/broadcast/post_broadcast.py

@@ -3,6 +3,7 @@ from itertools import chain
 from flask import current_app, jsonify, request
 from notifications_utils.polygons import Polygons
 from notifications_utils.template import BroadcastMessageTemplate
+from sqlalchemy.orm.exc import MultipleResultsFound
 
 from app import api_user, authenticated_service
 from app.broadcast_message.translators import cap_xml_to_dict
@@ -105,10 +106,17 @@ def create_broadcast():
 
 
 def _cancel_or_reject_broadcast(references_to_original_broadcast, service_id):
-    broadcast_message = dao_get_broadcast_message_by_references_and_service_id(
-        references_to_original_broadcast,
-        service_id
-    )
+    try:
+        broadcast_message = dao_get_broadcast_message_by_references_and_service_id(
+            references_to_original_broadcast,
+            service_id
+        )
+    except MultipleResultsFound:
+        raise BadRequestError(
+            message='Multiple alerts found - unclear which one to cancel',
+            status_code=400,
+        )
+
     if broadcast_message.status == BroadcastStatusType.PENDING_APPROVAL:
         new_status = BroadcastStatusType.REJECTED
     else: