Merge r1745039 from trunk:

include the scheme in the error message about missing a 
mod_proxy_submodule (mod_proxy_$usually_a_scheme)


Submitted by: covener
Reviewed by: jim, ylavic, icing


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1807773 13f79535-47bb-0310-9956-ffa450edef68