Attempt to add a new TOTP slot fails silently, when key is removed from USB before clicking `Ok`.

Steps to reproduce:

1. Make sure, LibremKey is plugged into USB.
2. Open LibremKeyAuthenticator.
3. When there's a valid QR-Code somewhere on the screen, click +.
4. A modal dialog window with new slot's name and secret is displayed.
5. Remove the key from USB slot.
6. Click Ok on the dialog box, confirming that user wants to save the new TOTP slot. Since the key is not in USB slot, operation fails, but there's no feedback about that failure to the user.

In this scenario, app has to display an error message, notifying user about the failure to save the new slot.