Bug 1673609

Description of problem:
ip(6)tables-save (nf_tables version) reports error for uninitialized table instead of empty/valid output if the table is not present in nf_tables.
The error is reported on standard output instead of standard error one.
If not specific table is queried via --table, the tool exits correctly without any messages and with 0 exit code.

On the contrary, iptables binary initializes queried tables in background when in --list mode thus always producing at least default output.

Version-Release number of selected component (if applicable):
iptables-1.8.2-5.el8.x86_64

How reproducible:
always

Steps to Reproduce:
ip6tables-save; echo EC=$?
EC=0
ip6tables-save -t raw 2> >(sed 's/^/err: /') > >(sed 's/^/out: /'); echo EC=$?
out: Table `raw' does not exist
EC=1
ip6tables-save -t foo 2> >(sed 's/^/err: /') > >(sed 's/^/out: /'); echo EC=$?
out: Table `foo' does not exist
EC=1
ip6tables -t raw -L
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         

ip6tables-save -t raw 2> >(sed 's/^/err: /') > >(sed 's/^/out: /'); echo EC=$?
std: # Generated by xtables-save v1.8.2 on Thu Feb  7 10:35:27 2019
std: *raw
std: :PREROUTING ACCEPT [0:0]
std: :OUTPUT ACCEPT [0:0]
std: COMMIT
std: # Completed on Thu Feb  7 10:35:27 2019
EC=0

Actual results:
error is produced

Expected results:
ip(6)tables -t <valid_name> shows valid output/behaves the same as old iptables.
if stdout is used, the error should be after '#' which is ignored by '-restore' variant of command.

use of invalid table name is indistinguishable from valid name if the valid name is not present in nft ruleset

Additional info: