[Tunnelbroker] Remove usages of unwrap and expect

Summary:
Unwrap and expect were convenient for the happy cases. However,
they are unsatisfactory for actual edge cases as they cause panic which
puts tunnelbroker into a semi-broken state.

Depends on D7800

Test Plan:

nix develop

# init localstack and run terraform
comm-dev services start
(cd services/terraform && ./run.sh)

(cd services/tunnelbroker && cargo run &)
(cd services/commtest && cargo test --test tunnelbroker_integration_test)

Reviewers: bartek, varun

Reviewed By: bartek, varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7801