test app: Improve status and error handling a bit

- Reset "running" check box if an error occurs
- Add check box to manually reset position source's error code, as error
  is not emitted with NoError
- There is no need to handle the "OK" case in errorChanged, as the
  corresponding signal will not be emitted for NoError

Change-Id: I53d637c8efd807d6c3e8abff98963585fe74f220
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>