Diffusion - nuke faulty javascript error handler

Summary: Fixes T3913.

Test Plan: messed around with a comment on a commit and saw preview still updating correctly

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T3913

Differential Revision: https://secure.phabricator.com/D10382