ProxyErrorOverride inside location

[Nick Gearls]

A patch is submitted: 
https://issues.apache.org/bugzilla/show_bug.cgi?id=46656

Here are the modifs:
- added |ACCESS_CONF to AP_INIT_FLAG
- error_override & error_override_set are moved to proxy_dir_conf
- set_proxy_error_override() is modified to use provided dconf*
As I explained, this is very useful when using Web Services, as SOAP 
errors are reported inside status 500 pages.

Will this be added ?