Improve Device validation error messages

[GDYendell]

Currently when validation fails for a Device with parents it is not clear which yaml file the failure is in.

We could catch the error in Device.deserialize and add some context, but this would be too late to tell which yaml file the problem is in because it is all loaded into one dictionary at this point. Perhaps Device.deserialize_parents should validate each as it is loaded and fail at that point.

[gilesknap]

I approve this message.