Show user warning if receive quick lookup fails

just b/c a UPC doesn't exist yet doesn't prevent the batch from (in
some cases) adding a row for "unknown product" - but if the UPC is
sufficiently invalid, that can't happen

tailbone/api/batch/core.py

@@ -333,6 +333,9 @@ class APIBatchRowView(APIBatchMixin, APIMasterView):
                 msg = "Feature is not implemented"
             return {'error': msg}
 
+        if not row:
+            return {'error': "Could not identify product"}
+
         self.Session.flush()
         result = self._get(obj=row)
         result['ok'] = True