Fix fatal when deleted user is subscribed to a task and we generate an email to them

Summary: Fixes T3528. We won't be able to load the user if they've been deleted, and will fatal a few lines later on `$user->getID()`.

Test Plan: I'm going with my gut on this one.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3528

Differential Revision: https://secure.phabricator.com/D6442