Improve error handling of SMART STATUS if output registers are unknown

[Christian Franke]

The ATA SMART RETURNS STATUS command provides the result in the ATA output registers. Buggy ATA/SATA drivers and SAT Layers often do not properly return the registers values.

If a SAT Layer does not provide an ATA Return Descriptor with the output registers, smartctl 5.39 prints the following message and aborts:

Error SMART Status command failed
Please get assistance from http://smartmontools.sourceforge.net/
A mandatory SMART command failed: exiting. To continue, add one or more '-T permissive' options.

Smartctl should not abort in this case and print a more specific warning message. The output for 'smartctl -H ...' should be as follows:

SMART overall-health self-assessment test result: UNKNOWN

[Christian Franke]

r3081.