try to handle some 404 urls if possible

see https://github.com/GigaOM/gigaom/issues/4854
Crap, no. [Canonical redirects happen at the default priority](https://core.trac.wordpress.org/browser/tags/3.9.1/src/wp-includes/canonical.php#L530), and we don't want to precede those. Maybe this should go to 11?