FIX: Purge deprecated exception content accesses

[effigies]

Summary

Apparently this was deprecated in Python 2.6 and we missed it. BaseException.__str__ is the generic way to access messages.

Fixes #3270.

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

• Remove uses of BaseException.message; Update to BaseException.str`
• Remove uses of OSError.__getitem__; Update to OSError.errno

Acknowledgment

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

[codecov]

Codecov Report

Merging #3272 (77913ee) into master (7d538fa) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3272      +/-   ##
==========================================
+ Coverage   64.98%   65.02%   +0.03%     
==========================================
  Files         302      302              
  Lines       39947    39947              
  Branches     5283     5283              
==========================================
+ Hits        25961    25974      +13     
+ Misses      12910    12902       -8     
+ Partials     1076     1071       -5     

Flag	Coverage Δ
unittests	64.94% <0.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
nipype/interfaces/io.py	56.41% <0.00%> (ø)
nipype/utils/filemanip.py	72.94% <0.00%> (ø)
nipype/utils/subprocess.py	86.36% <0.00%> (ø)
nipype/pipeline/plugins/base.py	59.72% <0.00%> (+1.64%)	⬆️
nipype/pipeline/plugins/legacymultiproc.py	69.11% <0.00%> (+3.43%)	⬆️

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[effigies]

Ready for review.