[NCCL][Blocking Wait] Log set exceptions when checking for exceptions in (#54558)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54558
In blocking wait's polling synchronization loop, we frequently call checkAndSetException() as part of isCompleted() to check the status of nccl operations. It would be useful to log here in case we encounter any exceptions (which are later thrown by `checkAndThrowException`).
Also slightly refactors code previously added to make use of a helper function to get the error message given an `std::exception_ptr`.
ghstack-source-id: 125124314
Test Plan: CI
Reviewed By: pritamdamania87
Differential Revision: D27136202
fbshipit-source-id: 256eb63c5c2a84be909722d3fd7377ad9303fa11
