Do not cache 404s

We are caching 404s and this probably makes the 'time to index' problem worse. In other words if we cache the first 404 for a few minutes then even if the item gets in the index immediately afterwards then it might still 404.

you can see this happening by `curl -v "http://www.theguardian.com/its-broken" > /dev/null` a couple of times and look at the Age and Cache-Hits headers.

This is caching by omission (i.e. not setting cache headers). If we want to cache 404s we need to set appropriate headers.