cmd/mgmt/device/devicex: make error message more accurate
cmd/mgmt/device/devicex: make error message more accurate

If -d $VANADIUM_DEVICE_DIR fails, it could be because it doesn't exist.

Change-Id: Iae72c2309839fcc9d1538ae9b2e1f07176065fcf

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref