mod_proxy: follow up to r1681694.

Handle the proxy-error-override note also in mod_proxy_ajp.

The note is not needed in mod_proxy_fcgi (which also handles
ProxyErrorOverride) since it calls ap_die() by itself, and always
returns OK to proxy_handler().

Add a comment about the note where used.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1682907 13f79535-47bb-0310-9956-ffa450edef68