[FX] Better error when unpacking Proxy (#43740)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/43740
Test Plan: Imported from OSS
Reviewed By: suo
Differential Revision: D23380964
Pulled By: jamesr66a
fbshipit-source-id: 9658ef1c50d0f9c4de38781a7485002487f6d3f7
