mod_proxy_connect/wstunnel: If both client and backend sides get readable
at the same time, don't lose errors occuring while forwarding on the first
side when none occurs next on the other side, and abort.


CHANGES: Follow up to r1657636, clarify message.


mod_proxy_{connect,wstunnel}: follow up to r1657636.
Since rv is not used as an apr_status_t in the loop (it's logging is done by
proxy_{connect,wstunnel}_transfer() when necessary/relevent), use a boolean
instead to avoid the ugly ORs between APR error codes.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1670324 13f79535-47bb-0310-9956-ffa450edef68