xtables-restore: Extend failure error message

If a line causes zero 'ret' value and errno is set, call nft_strerror()
for a more detailed error message. While not perfect, it helps with
debugging ominous "line NN failed" messages pointing at COMMIT:

| # iptables-nft-restore <<EOF
| *filter
| -A nonexist
| COMMIT
| EOF
| iptables-nft-restore: line 3 failed: No chain/target/match by that name.

Signed-off-by: Phil Sutter <phil@nwl.cc>

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