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/DiffusionPathValidateController.php

@@ -2,12 +2,11 @@
 
 final class DiffusionPathValidateController extends DiffusionController {
 
-  public function willProcessRequest(array $data) {
-    // Don't build a DiffusionRequest.
+  protected function shouldLoadDiffusionRequest() {
+    return false;
   }
 
-  public function processRequest() {
-    $request = $this->getRequest();
+  protected function processDiffusionRequest(AphrontRequest $request) {
 
     $repository_phid = $request->getStr('repositoryPHID');
     $repository = id(new PhabricatorRepositoryQuery())