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