filters can now report an HTTP error to the server. This is done

filters can now report an HTTP error to the server.  This is done
by sending a brigade where the first bucket is an error_bucket.
This bucket is a simple bucket that stores an HTTP error and
a string.  Currently the string is not used, but it may be needed
to output an error log.  The http_header_filter will find this
bucket, and output the error text, and then return
AP_FILTER_ERROR, which informs the server that the error web page
has already been sent.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87863 13f79535-47bb-0310-9956-ffa450edef68