mod_proxy: Respect errors reported by pre_connection hooks.

MFC: 111304 (adjusted by Justin to apply against 2.0.x proxy)
Reviewed by: trawick, jerenkrantz, fielding


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.0.x@151375 13f79535-47bb-0310-9956-ffa450edef68