mod_proxy_ajp: Add support for 'ProxyErrorOverride on'.

PR: 50945
Submitted by: Peter Pramberger <peter pramberger.at>, jim
Reviewed by: igalic, rpluem
Backports: r1087864



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1374375 13f79535-47bb-0310-9956-ffa450edef68