DEBUG: mux-spop: Review some trace messages to adjust the message or the level

DEBUG: mux-spop: Review some trace messages to adjust the message or the level

Some trace messages were not really accurrate, reporting a CLOSED connection
while only an error was reported on it. In addition, an TRACE_ERROR() was
used to report a short read on HELLO/DISCONNECT frames header. But it is not
an error. a TRACE_DEVEL() should be used instead.

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

(cherry picked from commit e2ae8a74e87d32730e3edba26626518fd33e3a58)
Signed-off-by: Willy Tarreau <w@1wt.eu>