Flush early when populating batch, to ensure error is shown

tailbone/views/batch/core.py

@@ -1057,6 +1057,7 @@ class BatchMasterView(MasterView):
         user = session.query(model.User).get(user_uuid)
         try:
             self.handler.do_populate(batch, user, progress=progress)
+            session.flush()
         except Exception as error:
             session.rollback()
             log.exception("batch population failed: %s", batch)