MINOR: session: Rely on client source address at session level to log error

MINOR: session: Rely on client source address at session level to log error

When an embryonic session is killed, if no log format is defined for this
error, a generic error is emitted. When this happens, we now rely on the
session to get the client source address. For now, session addresses are
never set. So, thanks to the fallback mechanism, no changes are expected
with this patch. But its purpose is to rely on addresses at the session
level when set instead of those at the connection level.