Set the Default Time-to-Live (TTL) for failed requests.
Certain types of failures (such as "connection refused" and
"404 Not Found") are able to be negatively-cached for a short time.
Modern web servers should provide Expires: header, however if they
do not this can provide a minimum TTL.
The default is not to cache errors with unknown expiry details.
Note that this is different from negative caching of DNS lookups.
WARNING: Doing this VIOLATES the HTTP standard. Enabling
this feature could make you liable for problems which it
causes.