http_header_filter: on check_headers() failure, use AP_FILTER_ERROR and EOC

semantics to respectively warn the caller and cleanly terminate the connection
afterwards.

Suggested by: rpluem



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1775195 13f79535-47bb-0310-9956-ffa450edef68