fix: lack of result file raises internal error. closes #3088

[satra]

Summary

Fixes #3088 .

List of changes proposed in this PR (pull-request)

trap and return None when result file does not exist after a node exception in multiproc.

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[codecov]

Codecov Report

Merging #3142 into master will increase coverage by 0.03%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #3142      +/-   ##
==========================================
+ Coverage   67.59%   67.62%   +0.03%     
==========================================
  Files         299      299              
  Lines       39480    39483       +3     
  Branches     5220     5220              
==========================================
+ Hits        26685    26699      +14     
+ Misses      12082    12072      -10     
+ Partials      713      712       -1

Flag	Coverage Δ
#smoketests	53.01% <0%> (-0.01%)	⬇️
#unittests	64.86% <50%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
nipype/pipeline/plugins/multiproc.py	80.36% <50%> (-0.89%)	⬇️
nipype/interfaces/dcm2nii.py	64.78% <0%> (+6.1%)	⬆️

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