[RFC] use sigterm instead of sigkill to kill the server

sys.exit() causes a hang for some reason but
os._exit() does not

I'd rather know why sys.exit() hangs and has to be sigkilled of course. I didn't try very hard to find out but I thought others might have a clue. Hence, RFC.
I'd remove the `AttributeError` from here, as this upnp wrapper sometimes might emit some unknown errors, and we're just silently not choking on them.