merge 1592032 from trunk:

mod_proxy_fcgi: better error messages, and fix an old bug where the error
                condition was forgotten

Submitted by: trawick
Reviewed by: ylavic, covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1604369 13f79535-47bb-0310-9956-ffa450edef68