Time-to-Live (TTL) for failed requests.  Certain types of
	failures (such as "connection refused" and "404 Not Found") are
	negatively-cached for a configurable amount of time.  The
	default is 5 minutes.  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.