mod_proxy: Ensure network errors detected by the proxy are returned as

504 Gateway Timout as opposed to 502 Bad Gateway, in order to be
compliant with RFC2616 14.9.4 Cache Revalidation and Reload Controls.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1480058 13f79535-47bb-0310-9956-ffa450edef68