[emocam] No indication if no webcam is found

If you try to use emocam with no webcam being detected, emocam will not say anything, other than Qt(?) throwing an error in the console:
```
[10/24 19:39:57] [DEBUG] [qml] [FlickableWebViewCore.qml] Web Entity JS message: https://more.overte.org/applications/emocam/index.html?1698168991678 0 Uncaught (in promise) NotFoundError: Requested device not found
```