ENH: raise exception if dcm2niix returns with non-0 code

[yarikoptic]

apparently we do not fail whenever dcm2niix fails. I think it might have been some change in behavior, may be in nipype?

apparently there is odd difference in nipype invocation of dcm2niix -- it doesn't fail whenever exits with 1: https://github.com/nipy/nipype/issues/3592 .

TODOs:

• [ ] remove pdb invocation
• [ ] add a test

[codecov[bot]]

Codecov Report

Patch coverage: 20.00% and project coverage change: -0.08 :warning:

Comparison is base (83c8424) 82.00% compared to head (7904d46) 81.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #695      +/-   ##
==========================================
- Coverage   82.00%   81.93%   -0.08%     
==========================================
  Files          41       41              
  Lines        4130     4135       +5     
==========================================
+ Hits         3387     3388       +1     
- Misses        743      747       +4     

Impacted Files	Coverage Δ
heudiconv/convert.py	84.71% <20.00%> (-0.83%)	:arrow_down:

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