Fix a fatal when viewing a daemon task with an invalid ID

Summary:
Fixes T11295. Prior to this change, the "404 page" for daemon tasks fatals.

This page is special cased a little bit and not a normal 404 page, because it's possible for you to click a valid link and the task to get GC'd by the time you load the page, or similar. It tries to be a little more user-friendly than a bare 404.

Test Plan:

• Visited /daemon/task/1428348920328/ (any invalid ID).
• Now got a nice "no such task" page.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11295

Differential Revision: https://secure.phabricator.com/D16254