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.