main: propagate error to shell

main: propagate error to shell

Before:

 # nft add rule ip test input ip hdrlength 3
 <cmdline>:1:1-37: Error: Could not process rule: Invalid argument
 add rule ip test input ip hdrlength 3
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 # echo $?
 0

After:

 # nft add rule ip test input ip hdrlength 3
 <cmdline>:1:1-37: Error: Could not process rule: Invalid argument
 add rule ip test input ip hdrlength 3
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 # echo $?
 1

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