[jit] fix segfault on missing getstate (#32642)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32642
Previously, if we defined `__setstate__` but not `__getstate__`, we
would segfault. This PR turns that into a comprehensible error message
(and improves another error message as well).
Fixes https://github.com/pytorch/pytorch/issues/25886
Test Plan: Imported from OSS
Differential Revision: D19596463
Pulled By: suo
fbshipit-source-id: dbe76bc36bc747d65fb0223184c009e0e9ba072c
