docs: errors messages may describe desired state

[irbull]

Update the CONTRIBUTING.md to indicate that error messages should describe the desired state if possible.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.25%. Comparing base (2a0f511) to head (ca5ecc3). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5757 +/- ## ======================================= Coverage 96.25% 96.25% ======================================= Files 475 475 Lines 38475 38475 Branches 5587 5587 ======================================= Hits 37036 37036 Misses 1397 1397 Partials 42 42 ```

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