tools: ynl: use strerror() if no extack of note provided

If kernel didn't give use any meaningful error - print
a strerror() to the ynl error message.

Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20231202211310.342716-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>