More informative AltConstraint error

[bpoldrack]

Closes #7092

[bpoldrack]

crippledFS failure: https://github.com/datalad/datalad/actions/runs/3376700932/jobs/5604739627

This one is strange. Apart from the fact that I don't see even a remote relationship to change here (and it didn't happen in #7115), I'm also wondering how the test could ever pass on crippled FS before. It relies on checking out DEFAULT_BRANCH which shouldn't work that way on adjusted branches.
I'm missing something. ATM neither the fact that it didn't fail before nor the fact that it now starts to fail makes any sense to me.

[codeclimate]

Code Climate has analyzed commit 9208cf8 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Base: 89.17% // Head: 90.81% // Increases project coverage by +1.64% 🎉

Coverage data is based on head (9208cf8) compared to base (b25dd03).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7132      +/-   ##
==========================================
+ Coverage   89.17%   90.81%   +1.64%     
==========================================
  Files         325      325              
  Lines       44095    44095              
  Branches     5853     5853              
==========================================
+ Hits        39321    40046     +725     
+ Misses       4759     4034     -725     
  Partials       15       15              

Impacted Files	Coverage Δ
datalad/support/constraints.py	91.32% <100.00%> (ø)
datalad/_version.py	45.68% <0.00%> (+0.27%)	⬆️
datalad/__init__.py	98.00% <0.00%> (+16.00%)	⬆️
datalad/tests/utils.py	56.96% <0.00%> (+35.36%)	⬆️
datalad/tests/test_tests_utils.py	92.34% <0.00%> (+92.34%)	⬆️

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[bpoldrack]

I think this is trivial. Since CI passes now, will go ahead and merge.

[yarikoptic-gitmate]

PR released in 0.18.0