Make a specific cache for 404 errors

This prevent the home page to send a 404 status instead of a 200.

Create a "404.error.php" file into the cache directory to store the result of a "page not found" result.

This PR can also be merged in Novius OS Dubrovka.
Prepend cache path to the 404 error prefix

to allow multiple 404 pages in cache
Merge pull request #103 from Foine/patch-5

Make a specific cache for 404 errors