CoreWLANWirelessManager should gracefully exit if no WiFi interface is available on host

[Manish Sinha]

When there is no WiFi interface present on the host, interfacesPicker.AddItems(CWInterface.InterfaceNames) throws System.ArgumentNullException as CWInterface.InterfaceNames.

This is not the best experience for a sample and we need to gracefully exit with an error message.

Screenshot:

[Brad Umbaugh]

Thanks @manish. Apologies for the slow response; this was fixed with #86.