[package] better error message when unpickling a mocked obj (#50159)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50159
Test Plan: Imported from OSS
Reviewed By: tugsbayasgalan
Differential Revision: D25809551
Pulled By: suo
fbshipit-source-id: 130587e650271cf158f5f5d9e688c622c9006631
