Provide a more informative alternative to 404 on invalid shortened Phurl URL

Summary: When accessing an invalid URL on the short Phurl domain, users should see informative message

Test Plan: Open URL in the previously configured Phurl short domain such as `https://www.zz.us` and see dialog with message. Open `https://www.zz.us/u/123` for a valid `U123` Phurl and access destination URL.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14450

src/applications/phurl/controller/PhabricatorPhurlShortURLDefaultController.php

@@ -8,6 +8,16 @@ final class PhabricatorPhurlShortURLDefaultController
   }
 
   public function handleRequest(AphrontRequest $request) {
-    return new Aphront404Response();
+    $dialog = $this->newDialog()
+      ->setTitle(pht('Invalid URL'))
+      ->appendParagraph(
+        pht('This domain can only be used to open URLs'.
+          ' shortened using the Phurl application. The'.
+          ' URL you are trying to access does not have'.
+          ' a Phurl URL associated with it.'));
+
+    return id(new AphrontDialogResponse())
+      ->setDialog($dialog)
+      ->setHTTPResponseCode(404);
   }
 }