[RPC] Add exception logging to constValue() (#67802)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67802
In RPC C++ code, we might sometimes call constValue() when the future actually has an exception, and in unittests we want to assert on the exception. What happens is that we get a message basically saying "!eptr_" which indicates there is some exception but we don't know what it is.
This diff simply adds logging for the exception and mentions that `value` over `constValue` should be used when the future can have an exception. The contract of `constValue` to throw when `eptr_` is set is still held, it is just enhanced with additional logging.
ghstack-source-id: 142375391
Test Plan: Added UT
Reviewed By: mrshenli
Differential Revision: D32156552
fbshipit-source-id: 4dd5e73b92173209074c104a4b75c2021e20de4b
