Disable sending of error-notes on a 500 (Internal Server Error) response

since it often includes file path info.  Enable sending of error-notes
on a 501 (Method Not Implemented).

http_config.c would respond with 501 (Method Not Implemented) if a
content type handler was specified but could not be found, which
should have been a 500 response.  Likewise, mod_proxy.c would responsd
with a 501 if the URI scheme is unrecognized instead of the correct
response of 403 (Forbidden).

PR: 3173


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82333 13f79535-47bb-0310-9956-ffa450edef68