Support custom ErrorDocuments for HTTP 501 and 414 status codes.

PR 57167 [Edward Lu <Chaosed0 gmail.com>]

Submitted By: Edward Lu <Chaosed0 gmail.com>
Committed By: covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1635762 13f79535-47bb-0310-9956-ffa450edef68