Refactored DatalessError to indicate implementation is now mature

[ESadek-MO]

🚀 Pull Request

Description

A better approach than #6864.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.43%. Comparing base (2586e00) to head (0d76f09).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/cube.py	0.00%	2 Missing ⚠️
lib/iris/_lazy_data.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6865   +/-   ##
=======================================
  Coverage   90.42%   90.43%           
=======================================
  Files          91       91           
  Lines       24841    24839    -2     
  Branches     4656     4656           
=======================================
  Hits        22463    22463           
+ Misses       1610     1608    -2     
  Partials      768      768           

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[ukmo-ccbunney]

All looks splendiferous.
LGTM.