[rpc] create error string in listenLoop outside of lock (#35393)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35393
this was being created inside the lock scope, but we don't need to
hold the lock for this.
ghstack-source-id: 100953426
Test Plan: CI
Differential Revision: D20632225
fbshipit-source-id: dbf6746f638b7df5fefd9bbfceaa6b1a542580e2
