[Keyserver/rust] Improve debugging

Summary:
This was useful in helping track down
https://linear.app/comm/issue/ENG-4418. Would like to see these changes
persisted so if errors do arise in the future, we get more than nothing

Test Plan:

cd keyserver
RUST_LOG=debug yarn dev

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8587