Device health via smartctl status code

[ercpe]

See #5 (comment) and the comment in __main__.py:

# TODO: See if we can get hints from the smartctl exit codes:
# https://www.freebsd.org/cgi/man.cgi?query=smartctl&manpath=FreeBSD+9.0-RELEASE+and+Ports&format=html#RETURN_VALUES
# at least we should not handle if the drive is in low-power mode (spindown)

• Make sure that the status codes are stable across platforms and versions of smartmontools
• Implement it as an additional check