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

src/applications/diffusion/controller/DiffusionRepositoryDefaultController.php

@@ -2,7 +2,7 @@
 
 final class DiffusionRepositoryDefaultController extends DiffusionController {
 
-  public function processRequest() {
+  protected function processDiffusionRequest(AphrontRequest $request) {
     // NOTE: This controller is just here to make sure we call
     // willBeginExecution() on any /diffusion/X/ URI, so we can intercept
     // `git`, `hg` and `svn` HTTP protocol requests.