Report subprocess error messages

[aarmaane]

Closes #50. I planned on doing this along with my first PR, but I forgot about it until now.

I appended the error message as-is to the MadoopError. This risks making the error messages a bit cluttered though, so any ideas for a cleaner format are welcome.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.98%. Comparing base (042f54b) to head (30214c9).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
madoop/mapreduce.py	66.66%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #81      +/-   ##
===========================================
- Coverage    98.43%   96.98%   -1.46%     
===========================================
  Files            4        4              
  Lines          256      265       +9     
===========================================
+ Hits           252      257       +5     
- Misses           4        8       +4     

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[awdeorio]

Thanks for the contribution! A quick look seems like this is a reasonable approach and thanks for adding tests. I'll take a closer look soon.

[awdeorio]

LGTM after pushing some changes. Take a look and let me know if they look OK to you.

[aarmaane]

Pointed out a style choice and added your new messages to the tests. Otherwise, LGTM.