Make ErrorDocuments work for proxy error messages.

Define "error-notes" as a  communication channel modules->core

PR: 494, 1259
Reviewed by:	Dean Gaudet, Marc Slemco, Chuck Murcko


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81375 13f79535-47bb-0310-9956-ffa450edef68