Comment 2 for bug 1988749

I agree it should fail in such situations. But at this point, some people might have invalid interface names in their configuration, and as it's not failing it's not causing problems. If we switch the behavior to fail-by-default, we might cause real problems out there.

I propose we should emit a warning initially so users can be aware they have a problem in their configuration: https://github.com/canonical/netplan/pull/313