core: Simpler connection close logic if discarding the request body fails.

If ap_discard_request_body() sets AP_CONN_CLOSE by itself it simplifies and
allows to consolidate end_output_stream() and error_output_stream().



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1898683 13f79535-47bb-0310-9956-ffa450edef68
