more standard shape for error messages in combinat

[fchapoton]

📚 Description

This changes most (but not all) error messages in combinat/ to conform to the Python convention (no capital, no final dot)

📝 Checklist

• I have made sure that the title is self-explanatory and the description concisely explains the PR.
• I have linked an issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[codecov-commenter]

Codecov Report

Patch coverage: 86.76% and project coverage change: +0.01 🎉

Comparison is base (52a81cb) 88.57% compared to head (0e3557b) 88.59%.

❗ Current head 0e3557b differs from pull request most recent head 7688b96. Consider uploading reports for the commit 7688b96 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #35252      +/-   ##
===========================================
+ Coverage    88.57%   88.59%   +0.01%     
===========================================
  Files         2140     2140              
  Lines       397273   397415     +142     
===========================================
+ Hits        351891   352078     +187     
+ Misses       45382    45337      -45     

Impacted Files	Coverage Δ
src/sage/combinat/binary_tree.py	97.08% <0.00%> (ø)
src/sage/combinat/combinat.py	93.57% <0.00%> (ø)
src/sage/combinat/combinatorial_map.py	94.11% <0.00%> (ø)
src/sage/combinat/composition.py	97.37% <0.00%> (ø)
src/sage/combinat/constellation.py	91.59% <0.00%> (+0.40%)	⬆️
src/sage/combinat/e_one_star.py	90.84% <0.00%> (ø)
src/sage/combinat/fully_packed_loop.py	96.03% <ø> (ø)
src/sage/combinat/interval_posets.py	93.93% <ø> (ø)
src/sage/combinat/parallelogram_polyomino.py	97.08% <0.00%> (ø)
src/sage/combinat/partition.py	96.88% <0.00%> (ø)
... and 64 more

... and 27 files with indirect coverage changes

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[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 7688b96

[mkoeppe]

LGTM. The test failure (a timeout) is unrelated.