xtables-translate: Return non-zero if translation fails

xtables-translate: Return non-zero if translation fails

Untranslated parts in output are easily overlooked and also don't disrupt
piping into nft (which is a bad idea to begin with), so make a little
noise if things go sideways:

| # iptables-translate -A FORWARD -m recent --set
| nft # -A FORWARD -m recent --set
| Translation not (fully) implemented

| # cat /tmp/input.ipt
| *filter
| -A FORWARD -s 1.2.3.4
| -A FORWARD -m recent --set
| COMMIT
| # iptables-restore-translate -f /tmp/input.ipt
| # Translated by iptables-restore-translate v1.8.13 on Tue Mar 10 17:29:17 2026
| add table ip filter
| add rule ip filter FORWARD ip saddr 1.2.3.4 counter
| # -t filter -A FORWARD -m recent --set
| iptables-translate-restore: line 3 failed

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>