Also ensure that the error message is actually printed to stderr and that
the rule generation is aborted if an interface cannot be resolved.
Ref: https://github.com/openwrt/luci/issues/3975
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 559b3384666bbc6e4e9e6d86cf54bd88d30b341f)