Log warning instead of error for batch population error

this is most typically caused by bad user input; a warning is shown on
screen so they hopefully can guess what the problem is.  no need to
loop in the admins via email

tailbone/views/batch/core.py

@@ -1057,7 +1057,8 @@ class BatchMasterView(MasterView):
             session.flush()
         except Exception as error:
             session.rollback()
-            log.exception("population failed for batch %s: %s", batch.uuid, batch)
+            log.warning("population failed for batch %s: %s", batch.uuid, batch,
+                        exc_info=True)
             session.close()
             if progress:
                 progress.session.load()