Return 400 if references missing from cancel broadcast

If someone tries to cancel a broadcast but the references don’t match
and existing broadcast we correctly return a 404.

If they don’t provide any references then we get an exception. This
commit catches the missing references and returns a 400. I think this
is more appropriate because it’s malformed request, rather than a
well-formed request that doesn’t match our data. It also lets us write a
more specific and helpful error message.

app/v2/broadcast/post_broadcast.py

@@ -48,6 +48,11 @@ def create_broadcast():
     validate(broadcast_json, post_broadcast_schema)
 
     if broadcast_json["msgType"] == "Cancel":
+        if broadcast_json["references"] is None:
+            raise BadRequestError(
+                message='Missing <references>',
+                status_code=400,
+            )
         broadcast_message = _cancel_or_reject_broadcast(
             broadcast_json["references"].split(","),
             authenticated_service.id