Device Missing

[markwallsgrove]

usb.findByIds will return undefined if the device is not plugged in. The second line in the snippet below causes a error to be thrown TypeError: Cannot read property 'open' of undefined.

    device = usb.findByIds(this.vid, this.pid);
    device.open();

Would it be possible it either return a error to the callback or throw a exception? If you could state your preference I don't mind raising a PR.

[dave-irvine]

@markwallsgrove Sorry for some reason I never got notified about this issue.

I'd prefer to return an error to the callback.