(stable) Recover gracefully from Conduit failure when building "Tags" field in commit mail

Summary:
Ref T9458. This is basically the same as D13319, but the "Tags" field didn't get covered in that change.

Specifically, the issue is:

  - We try to generate mail to a disabled user (later, we'll drop it without delivering it, but that filtering doesn't happen yet).
  - The disabled user doesn't have permission to use Conduit (or any other Conduit-related problem occurs).
  - We fail here, then retry generating the mail again later.

Instead, just degrade to not building the field and showing what went wrong.

Test Plan:
  - Pushed some commits, saw mail generate.
  - Added a fake exception to the field, saw the mail generate with an error message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9458

Differential Revision: https://secure.phabricator.com/D14142

src/applications/repository/customfield/PhabricatorCommitTagsField.php

@@ -29,6 +29,7 @@ final class PhabricatorCommitTagsField
       'callsign' => $this->getObject()->getRepository()->getCallsign(),
     );
 
+    try {
       $tags_raw = id(new ConduitCall('diffusion.tagsquery', $params))
         ->setUser($this->getViewer())
         ->execute();
@@ -39,8 +40,12 @@ final class PhabricatorCommitTagsField
       }
       $tag_names = mpull($tags, 'getName');
       sort($tag_names);
+      $tag_names = implode(', ', $tag_names);
+    } catch (Exception $ex) {
+      $tag_names = pht('<%s: %s>', get_class($ex), $ex->getMessage());
+    }
 
-    $body->addTextSection(pht('TAGS'), implode(', ', $tag_names));
+    $body->addTextSection(pht('TAGS'), $tag_names);
   }
 
 }