iptables: refer to dmesg when we hit error

iptables: refer to dmesg when we hit error

This does not make any better, but at least refer to
dmesg which is the common source of information to diagnose
kernel-side problems. This is helpful for newbie users.

# iptables -I INPUT -j CLUSTERIP
iptables: Invalid argument. Run `dmesg' for more information.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>