stages/disks: improve error reporting for LUKS device reuse
stages/disks: improve error reporting for LUKS device reuse

Report the specific reason that we declined to reuse a device.  Propagate
reuse errors earlier so we don't have to guess why reuse failed.

Fixes https://github.com/coreos/ignition/issues/1209.

coreos/ignition,coreos/ignition,coreos/ignition