src: improve error reporting when remove rules

src: improve error reporting when remove rules

 # nft delete rule ip y z handle 7
 Error: Could not process rule: No such file or directory
 delete rule ip y z handle 7
                ^

 # nft delete rule ip x z handle 7
 Error: Could not process rule: No such file or directory
 delete rule ip x z handle 7
                  ^

 # nft delete rule ip x x handle 7
 Error: Could not process rule: No such file or directory
 delete rule ip x x handle 7
                           ^

 # nft replace rule x y handle 10 ip saddr 1.1.1.2 counter
 Error: Could not process rule: No such file or directory
 replace rule x y handle 10 ip saddr 1.1.1.2 counter
                         ^^

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