Remove unnecessary error handling

The context here should be enough for the users, custom error
message is not needed.

app/v2/broadcast/post_broadcast.py

@@ -101,17 +101,10 @@ def create_broadcast():
 
 
 def _cancel_or_reject_broadcast(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 NoResultFound:
-        raise BadRequestError(
-            message="Broadcast message reference and service id didn't match with any existing broadcasts",
-            status_code=404,
-        )
-
+    broadcast_message = dao_get_broadcast_message_by_references_and_service_id(
+        references_to_original_broadcast,
+        service_id
+    )
     if broadcast_message.status == BroadcastStatusType.PENDING_APPROVAL:
         new_status = BroadcastStatusType.REJECTED
     else:

tests/app/v2/broadcast/test_post_broadcast.py

@@ -234,9 +234,6 @@ def test_cancel_request_does_not_cancel_broadcast_if_reference_does_not_match(
     )
 
     assert response_for_cancel.status_code == 404
-    response = json.loads(response_for_cancel.get_data(as_text=True))
-    expected_error_message = "Broadcast message reference and service id didn't match with any existing broadcasts"
-    assert response["errors"][0]["message"] == expected_error_message
 
 
 def test_large_polygon_is_simplified(