mod_proxy_ajp: Rename output_failed to client_failed in ap_proxy_ajp_request().

Since any read/write error is caught by this flag, it really means "dialog with
client <ip:port> failed" (as per the associated log message), whereas write to
client (output) errors need not be handled differently than any error occuring
after some bytes have already been sent to the client, which is the purpose of
the data_sent flag.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1674056 13f79535-47bb-0310-9956-ffa450edef68