Improve Asana API error handling in Doorkeeper
Summary:
Ref T2852. We need to distinguish between an API call which worked but got back nothing (404) and an API call which failed.
In particular, Asana hit a sync issue which was likely the result of treating a 500 (or some other error) as a 404.
Also clean up a couple small things.
Test Plan: Ran syncs against deleted tasks and saw successful syncs of non-tasks, and simulated random failures and saw them get handled correctly.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2852
Differential Revision: https://secure.phabricator.com/D6470