HTTP/1.1 1000\n\r\n\r
would panic us trying to find a suitable message for 1000.

Now we 503 the response instead.

Fixes #506



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4052 d4fa192b-c00b-0410-8231-f00ffab90ce4