Diffusion - return 404 errors for bad URIs
Summary: Fixes T5646. Makes diffusion a much better user experience. Users now see a 404 exception page when they have a bad URI. Previously, they saw a developer-facing raw exception.
Test Plan: played around in diffusion a bunch. most of these changes were fairly mechanical at the end of the day.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T5646
Differential Revision: https://secure.phabricator.com/D11299