chromebook-setup: add some basic /dev check for storage
Sometimes the storage device specified may be "misdetected" as non-block one.
Say the USB device did not show up in /dev - faulty device, kernel did not detect it yet/properly ... or simply one did not plug it fully into the port.
Either way, add a trivial check which errors out when a /dev node is specified yet, it doesn't seems like a block device.