Fix halstead harvester for errors thrown by radon

[abadger]

Similar to the fix for cyclomatic complexity here:
7ba461c

the radon halstead harvester can return errors as a piece of data and we
need to log it rather than attempt to add it to our report.

[codecov-io]

Codecov Report

Merging #63 into master will decrease coverage by 0.21%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   99.46%   99.24%   -0.22%     
==========================================
  Files          20       20              
  Lines         930      933       +3     
==========================================
+ Hits          925      926       +1     
- Misses          5        7       +2

Impacted Files	Coverage Δ
wily/operators/halstead.py	93.1% <33.33%> (-6.9%)	⬇️

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[tonybaloney]

thanks @abadger I'd never come across errors in this harvester

[abadger]

Thanks for merging! I was running it from the root of my project and it turned up errors in some of my pip installed dependencies (urwid was one of them, i think). Once i could see the error, i started running wily on the specific directories that hold my code ;-)

[tonybaloney]

released in 1.12.2