[services] Tunnelbroker - Changes to use bail and ensure Anyhow macros

Summary:
This diff introduces a minor refactoring to use bail and ensure Anyhow macros.
This is a more idiomatic way to use Anyhow and makes the code more readable.

Test Plan: Service is successfully built and the Rust function is successfully called from the C++ side.

Reviewers: jon, michal, marcin, varun, tomek

Reviewed By: jon, marcin, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5262