Improve error message on invalid key:value format

[berland]

Issue
Resolves #12585

Approach
Rewrite text and adapt tests.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.56%. Comparing base (7bdb6d2) to head (7bee21b).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12586      +/-   ##
==========================================
- Coverage   90.58%   90.56%   -0.02%     
==========================================
  Files         435      435              
  Lines       29931    29933       +2     
==========================================
- Hits        27112    27110       -2     
- Misses       2819     2823       +4     

Flag	Coverage Δ
cli-tests	37.35% <ø> (-0.01%)	⬇️
gui-tests	68.66% <ø> (+0.01%)	⬆️
performance-and-unit-tests	73.92% <ø> (-0.02%)	⬇️
test	38.11% <ø> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #12586 will not alter performance

_{Comparing berland:improve_configvalidationerror_on_dicts (7bee21b) with main (e212883)}

Summary

✅ 22 untouched

[andreas-el]

💯