Fail more gracefully when rendering transcripts if handle is missing

Summary: Fixes T4628. I can only partially reproduce the root cause here, but if transcript display rules aren't quite right we should just degrade here rather than fatalling. Transcripts are a messy business by any measure.

Test Plan: Sort-of-reproing transcript renders OK now.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4628

Differential Revision: https://secure.phabricator.com/D8554

src/applications/herald/controller/HeraldTranscriptController.php

@@ -297,8 +297,10 @@ final class HeraldTranscriptController extends HeraldController {
         default:
           if ($target) {
             foreach ($target as $k => $phid) {
+              if (isset($handles[$phid])) {
                 $target[$k] = $handles[$phid]->getName();
               }
+            }
             $target = implode("\n", $target);
           } else {
             $target = '<empty>';