Update callsites of phutil_json_decode.

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

Test Plan: Ran the unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9641