Issue a proper 404 when trying to edit nonexistent events

Summary: We'll fatal a little later here when trying to call methods. 404 instead.

Test Plan: Visited `/calendar/event/edit/9999999/` or similar.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8591