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/DiffusionSymbolController.php

@@ -4,13 +4,9 @@ final class DiffusionSymbolController extends DiffusionController {
 
   private $name;
 
-  public function willProcessRequest(array $data) {
-    $this->name = $data['name'];
-  }
-
-  public function processRequest() {
-    $request = $this->getRequest();
+  protected function processDiffusionRequest(AphrontRequest $request) {
     $user = $request->getUser();
+    $this->name = $request->getURIData('name');
 
     $query = new DiffusionSymbolQuery();
     $query->setName($this->name);