MINOR: mux-spop: Set SPOP_CF_ERROR flag on connection error only

MINOR: mux-spop: Set SPOP_CF_ERROR flag on connection error only

The SPOP_CF_ERROR flag is now set on connection error only. It was also set
on some demux failures. But it is not mandatory because the connection is
closed anyway. And it is handy to have a flag dedicated to tcp connection
error. It was the original purpose of this flag.

This patch could be backported to 3.1 to ease future backports.

(cherry picked from commit 514a912a4d15c834a034be8ea7675c3ae0822080)
Signed-off-by: Willy Tarreau <w@1wt.eu>