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