fix: clean-up error message if only 1 ancestor is valid for ontology term

[nayib-jose-gloria]

Reason for Change

• cellxgene-schema must validate tissue_type #514
• See error message formatting feedback from Jason: cellxgene-schema must validate tissue_type #514 (comment)

Changes

• If we're only validating an ontology term against 1 allowed ancestor, print the ancestor term we're checking for in the error message as single term with single quotes instead of a list of 1.
• For scenarios where a list of ancestors is expected, print as a single concatenated list rather than a list of lists

Testing

• unit tests

Notes for Reviewer

[codecov]

Codecov Report

Merging #637 (a87075a) into main (6cef195) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
+ Coverage   83.13%   83.15%   +0.01%     
==========================================
  Files          19       19              
  Lines        1696     1698       +2     
==========================================
+ Hits         1410     1412       +2     
  Misses        286      286              

Flag	Coverage Δ
unittests	83.15% <100.00%> (+0.01%)	⬆️

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

Files Changed	Coverage Δ
cellxgene_schema_cli/cellxgene_schema/validate.py	93.72% <100.00%> (+0.02%)	⬆️

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