mod_proxy_fcgi: avoid loops serving proxied error documents

This commit should solve the issue indicated in PR 55415.
Httpd loops while serving a error document if:
1) The error document's content is proxied.
2) ProxyErrorOverride is set.
The solution proposed is to limit the use of ap_die only
to the initial request. I tested the change with very basic
scenarios but I am not sure if I got all the use cases,
feedback is really welcome.


Submitted by: elukey
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1757671 13f79535-47bb-0310-9956-ffa450edef68