MAINT: Refactor aggregate_outputs for readability

[oesteban]

Just a rewrite for improved readability.

Acknowledgment

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

[codecov-io]

Codecov Report

Merging #2969 into master will decrease coverage by 3.37%.
The diff coverage is 65.21%.

@@            Coverage Diff             @@
##           master    #2969      +/-   ##
==========================================
- Coverage   67.61%   64.23%   -3.38%     
==========================================
  Files         344      342       -2     
  Lines       43798    43746      -52     
  Branches     5471     5468       -3     
==========================================
- Hits        29612    28102    -1510     
- Misses      13475    14549    +1074     
- Partials      711     1095     +384

Flag	Coverage Δ
#smoketests	?
#unittests	64.23% <65.21%> (-0.8%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/base/core.py	84.46% <65.21%> (-2.45%)	⬇️
nipype/interfaces/nilearn.py	40% <0%> (-56.67%)	⬇️
nipype/utils/spm_docs.py	25.92% <0%> (-44.45%)	⬇️
nipype/interfaces/freesurfer/base.py	50% <0%> (-30.51%)	⬇️
nipype/utils/logger.py	59.7% <0%> (-29.86%)	⬇️
nipype/algorithms/rapidart.py	35% <0%> (-29.42%)	⬇️
nipype/interfaces/spm/base.py	57.94% <0%> (-29.14%)	⬇️
nipype/utils/provenance.py	55.73% <0%> (-28.35%)	⬇️
nipype/interfaces/fsl/model.py	55.26% <0%> (-25.35%)	⬇️
nipype/testing/fixtures.py	77.33% <0%> (-21.34%)	⬇️
... and 44 more

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[effigies]

Overall looks fine. I would strongly recommend against changing the exception types at this point, as it's just asking to break downstream code for very little benefit in clarity.

Two situations where you modify explicitly variables that would be modified implicitly by the functions that you pass them to. Feel free to reject those suggestions.