Catch more HTTP VCS errors and convert them into VCS repsonses

Summary:
Ref T13590. Currently, errors arising from cluster locking (like the "stuck write lock" exception) are not caught and converted into VCS responses on the HTTP VCS workflow.

Catch a broader range of exceptions and convert them into appropriate responses.

Test Plan:
  - Forced a "stuck write lock" exception, pushed to a Git repository over HTTP.
  - Before: generic fatal.
  - After: VCS-specific fatal with a useful message in the "X-Phabricator-Message" response header.

Maniphest Tasks: T13590

Differential Revision: https://secure.phabricator.com/D21525

src/applications/diffusion/controller/DiffusionServeController.php

@@ -397,7 +397,25 @@ final class DiffusionServeController extends DiffusionController {
       switch ($vcs_type) {
         case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
         case PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL:
-          $result = $this->serveVCSRequest($repository, $viewer);
+          $caught = null;
+          try {
+            $result = $this->serveVCSRequest($repository, $viewer);
+          } catch (Exception $ex) {
+            $caught = $ex;
+          } catch (Throwable $ex) {
+            $caught = $ex;
+          }
+
+          if ($caught) {
+            // We never expect an uncaught exception here, so dump it to the
+            // log. All routine errors should have been converted into Response
+            // objects by a lower layer.
+            phlog($caught);
+
+            $result = new PhabricatorVCSResponse(
+              500,
+              phutil_string_cast($caught->getMessage()));
+          }
           break;
         case PhabricatorRepositoryType::REPOSITORY_TYPE_SVN:
           $result = new PhabricatorVCSResponse(