Print a message if no --ethernet argument is supplied
Print a message if no --ethernet argument is supplied

If there is nowhere to listen for incoming connections, there is
no work to do. Rather than exit immediately and silently, this patch
prints a helpful message first.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit