Add Unsupported error for cases when storcli returns Un-supported.

[smoser]

The SAS3508 returns Un-supported when you ask it:

storcli /c0/dall show all
storcli /c0/vall show all

I'm not sure how we should query this card, but for the time being just add a new error type of Unsupported.

[codecov[bot]]

Codecov Report

Merging #99 (a1633e0) into master (77e1f1a) will increase coverage by 0.23%. The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   64.30%   64.54%   +0.23%     
==========================================
  Files          16       16              
  Lines        1706     1709       +3     
==========================================
+ Hits         1097     1103       +6     
+ Misses        441      439       -2     
+ Partials      168      167       -1     

Impacted Files	Coverage Δ
megaraid/megaraid.go	84.21% <ø> (ø)
linux/system.go	37.50% <50.00%> (+0.60%)	:arrow_up:
megaraid/storcli.go	67.50% <82.35%> (+0.96%)	:arrow_up:
linux/util.go	78.76% <0.00%> (+0.88%)	:arrow_up:

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[smoser]

@raharper adjusted, thank you for careful review.

[smoser]

Closing this in favor of #100.