Parse JSON as an associate array.

Summary:
It seems that in some situations, JSHint does not set the `evidence`
property. In such cases, PHP fails with `Undefined property:
stdClass::$evidence`. It would be safer to access the error object as an
associative array rather than as `stdClass`.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8993