[keyserver] Print exception that causes failed_to_retrieve_inbound_keys
Summary: This is useful for debugging. I needed it in order to diagnose ENG-7936, so I figured it would be good to add to the codebase in case we need to diagnose a similar issue in production in the future.
Test Plan: Confirm that relevant log was printed (see the Linear issue for the log)
Reviewers: will, varun, bartek
Reviewed By: bartek
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D11770