Update callsites of `phutil_json_decode`.

Summary: Depends on D9634. `phutil_json_decode` now throws an exception on invalid JSON.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9640