Avoid fatal checks in populate_operator_name (#1511)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1511

Make `populate_operator_name()` return an error so we can avoid crashing when there's a problem. Also ensure that the `overload` field is present before using it.

Reviewed By: larryliu0820

Differential Revision: D52451742

fbshipit-source-id: 85a5beb5e98db67534883fef36b9103d02a7b852