Improve error and exception handling for Drydock leases

Summary:
Ref T9252. See companion change in D14211. This does the same thing for leases.

Particularly, most of the TODOs about error handling can just be removed because they'll do the right things by default now.

This and D14211 also move slot lock release to after resource destruction. This feels cleaner than trying to release early at release/break.

Test Plan: Restarted a Harbormaster build, got a clean build result. This needs more vetting but I'll clean up any issues as I hit them.

Reviewers: chad, hach-que

Reviewed By: hach-que

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14212