mod_proxy: Print the correct error message for erroneous configured ProxyPass

directives. PR 40439.

(Backport of r441947)

Reviewed by: rpluem, jim, jerenkrantz


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@539277 13f79535-47bb-0310-9956-ffa450edef68