Minor change to 404 error caching

To help accommodate growing traffic patterns we are now caching 404 page errors. For the vast majority of WordPress sites hosted here, there will be no impact. However, if you have moved a page (made it a child of some other page) with out updating links to that page, those links will no longer work until you update the broken link to point to the actual URL.  If this change affects a large number of pages on your site, please contact askIT@yorku.ca.