[BUG] http: don't count req errors on client resets or t/o during keep-alive

[BUG] http: don't count req errors on client resets or t/o during keep-alive

We must set the error flags when detecting that a client has reset
a connection or timed out while waiting for a new request on a keep-alive
connection, otherwise process_session() sets it itself and counts one
request error.

That explains why some sites were showing an increase in request errors
with the keep-alive.