fastboot: Add some trivial error prefix for the getvar failure
fastboot: Add some trivial error prefix for the getvar failure

fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd