If -q flag is specified, do not complain when we are trying to delete
nonexistent NAT instance or nonexistent rule.

This allows execute batched `delete` commands and do not fail when
found nonexistent rule.

Restore the behaviour changed in r337536, when bad `ipfw delete` command
returns error.

Now -q option only makes it quiet. And when -f flag is specified, the
command will ignore errors and continue executing with next batched
command.