iptables-compat: homogenize error messages

iptables-compat: homogenize error messages

There are some differences between error messages in iptables and
iptables-compat:

 # iptables -C INPUT -s 192.168.2.102 -j ACCEPT
iptables: Bad rule (does a matching rule exist in that chain?).
 # iptables-compat -C INPUT -s 192.168.2.102 -j ACCEPT
iptables: No chain/target/match by that name.

 # iptables -N new_chain
 # iptables -N new_chain
iptables: Chain already exists.
 # iptables-compat -N new_chain
 # iptables-compat -N new_chain
iptables: File exists.

Now, iptables-compat shows the same error messages than iptables in
those cases.

Signed-off-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>