Add a status update to tell the user to connect a device

[jschanck]

Firefox relies on StatusUpdates to determine the UI that should be shown for a transaction. We send PresenceRequired if one device is connected, and we sends SelectDeviceNotice if multiple devices are connected. But there's currently no status update to indicate that no devices are connected.

[msirringhaus]

This would also be very helpful for interactively resetting an authenticator, to determine if the user has unplugged all devices, before starting the reset-flow.