[Identity] Improve VerifyUserAccessToken logging

Summary:
Was useful when tracking down where in this call something
was failing

In support of: https://linear.app/comm/issue/ENG-3977

Depends on D8891

Test Plan:

• Start identity service and tunnelbroker

cd services/commtest
cargo test --test identity_tunnelbroker_tests

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8916