[tunnelbroker] Fix error handling in main

Summary:
Address ENG-4795.
Done it by the way of rewriting most of Tunnelbroker code

Test Plan: Tunnelbroker still works as expected

Reviewers: kamil, will, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13612