mnl: extended error support for create command

mnl: extended error support for create command

 # nft create table x
 Error: Could not process rule: File exists
 create table x
              ^

 # nft create chain x y
 Error: Could not process rule: File exists
 create chain x y
                ^

 # nft create set x y { typeof ip saddr\; }
 Error: Could not process rule: File exists
 create set x y { typeof ip saddr; }
              ^

 # nft create counter x y
 Error: Could not process rule: File exists
 create counter x y
                  ^

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