Firmware updater should error out if missing Hello devices

[hello-binit]

If there are missing symlinks to Hello devices, the firmware updater will proceed with updating the devices that are present. In practice, most users see the "Congratulations!" success message at the end, and don't realize that some Hello devices were not updated.

To fix this issue, the firmware updater should error out on REx_firmware_updater.py --install at the beginning if any of the 6 Hello devices are missing.