Merge r1681694, r1682907 from trunk:

mod_proxy: Don't put the worker in error state for 500 or 503 errors
returned by the backend unless failonstatus is configured to.  PR 56925.


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.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1683112 13f79535-47bb-0310-9956-ffa450edef68