Add device diagnostics informations

[jahnf]

Give the user more information why a device could not be found. Or cannot be detected:

• It should be possible to get the following information via command line or the user interface:
  • No Logitech Spotlight device found.
  • Logitech Spotlight device found but cannot open/read (either wrong access rights, or the user is not member of the group that can access the device)
  • List of detected Spotlight devices
  • (optional / unsure: Check if the spotlight udev rules are valid)