Disable JSON shield for uncaught exception responses to Conduit requests

Summary: Modern conduit responses should never have a JSON shield. We disable it for normal responses, but uncaught exceptions hit this higher-level handler block which fails to disable the shield. Disable the shield.

Test Plan: Inspection.

Reviewers: btrahan, andrewjcg

Reviewed By: andrewjcg

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5483