When a worker task fails permanently, log the reason

Summary: Ref T6238. This makes debugging permanent task failures easier (we log reasoning for temporary failures already, just not permanent ones).

Test Plan: Saw more useful permanent failure log.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6238

Differential Revision: https://secure.phabricator.com/D10945