Refactor partitionByKey to get clearer error

[RustedBones]

Fix #5468

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.42%. Comparing base (5af9b53) to head (01d4d87).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5527      +/-   ##
==========================================
- Coverage   61.44%   61.42%   -0.02%     
==========================================
  Files         312      312              
  Lines       11105    11104       -1     
  Branches      744      757      +13     
==========================================
- Hits         6823     6821       -2     
- Misses       4282     4283       +1     

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