R1:1c088822b4d2

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.