[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