Make app exit on VPN server failure

[Darkren]

Did you run make format && make check?
Yes
Fixes #464

Changes:

• VPN server app now exists if Serve call failed
• VPN server now exists immediately if run on non-Linux OS

[jdknives]

@Darkren could we also make the VPN app shut down upon other fatal errors such as starting on MacOS?

[2020-08-12T18:42:26.542659+02:00] INFO (STDERR) [proc:vpn-server:0b7d29c85c5b489ea1e5ea4485f70499]: time="2020-08-12T18:42:26+02:00" level=info msg="Got app listener, bound to 44"
[2020-08-12T18:42:26.718356+02:00] INFO (STDERR) [proc:vpn-server:0b7d29c85c5b489ea1e5ea4485f70499]: time="2020-08-12T18:42:26+02:00" level=fatal msg="Error creating VPN server" error="error getting default network interface: server related methods are not supported for this OS"

But skywire-cli visor ls-apps returns the server as running and it is still shown as running in the hypervisor.