ENH: More verbose description when a faulty results file is loaded

[oesteban]

For reasons yet to be identified, some fMRIPrep (e.g. nipreps/fmriprep#1266) runs failed with:

Traceback (most recent call last):
  File "/usr/local/miniconda/lib/python3.7/site-packages/nipype/pipeline/engine/nodes.py", line 528, in _get_inputs
    output_value = results.outputs.trait_get()[output_name]
AttributeError: 'NoneType' object has no attribute 'trait_get'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/miniconda/lib/python3.7/site-packages/nipype/pipeline/plugins/multiproc.py", line 69, in run_node
    result['result'] = node.run(updatehash=updatehash)
  File "/usr/local/miniconda/lib/python3.7/site-packages/nipype/pipeline/engine/nodes.py", line 410, in run
    cached, updated = self.is_cached()
  File "/usr/local/miniconda/lib/python3.7/site-packages/nipype/pipeline/engine/nodes.py", line 296, in is_cached
    hashed_inputs, hashvalue = self._get_hashval()
  File "/usr/local/miniconda/lib/python3.7/site-packages/nipype/pipeline/engine/nodes.py", line 490, in _get_hashval
    self._get_inputs()
  File "/usr/local/miniconda/lib/python3.7/site-packages/nipype/pipeline/engine/nodes.py", line 530, in _get_inputs
    output_value = results.outputs.dictcopy()[output_name]
AttributeError: 'NoneType' object has no attribute 'dictcopy'

This PR identifies the situation earlier and provides more interpretable feedback.

[codecov-io]

Codecov Report

Merging #2920 into master will decrease coverage by 3.11%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2920      +/-   ##
==========================================
- Coverage   67.57%   64.45%   -3.12%     
==========================================
  Files         343      342       -1     
  Lines       43645    44386     +741     
  Branches     5428     5541     +113     
==========================================
- Hits        29494    28610     -884     
- Misses      13447    14687    +1240     
- Partials      704     1089     +385

Flag	Coverage Δ
#smoketests	?
#unittests	64.45% <50%> (-0.56%)	⬇️

Impacted Files	Coverage Δ
nipype/pipeline/engine/nodes.py	77.61% <50%> (-6.92%)	⬇️
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	58.08% <0%> (-29.05%)	⬇️
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 49 more

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[oesteban]

WDYT @satra? This may help diagnose lots of errors we are registering from fMRIPrep