Better 404 for Phame

Summary: "Fixes" fatals on phacility.com blog. If post_id is either 0 or any other integer not in the blog system, show a normal Phame 404 with crumbs.

Test Plan: http://local.blog.phacility.com/post/0/last_published/, http://local.blog.phacility.com/post/999999/last_published/

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16192