chore: change log error to warning (#61)

This shouldn't be logged as error since the CLI makes it clear that `ensure-group` will add a user to a group _if the group exists_.

stacklet/client/platform/cognito.py

@@ -92,5 +92,5 @@ def ensure_group(self, user, group) -> bool:
             self.log.debug(res)
             return True
         except self.client.exceptions.ResourceNotFoundException:
-            self.log.error("Group:%s doesn't exist.", group)
+            self.log.warning("Group:%s doesn't exist.", group)
             return False