refactor: validators return errors, not throw them

[jkowalleck]

the previous behavior of validators needed some improvements,
as test-integrations with the release candidate showed. ...

here they are now.

[codacy-production]

Coverage summary from Codacy

Merging #691 (7edc032) into main (080b63b) - See PR on Codacy

Coverage variation	Diff coverage
✅ 0.00% (target: -1.00%)	✅ 100.00% (target: 90.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (080b63b)	5902	5803	98.32%
Head commit (7edc032)	5898 (-4)	5799 (-4)	98.32% (0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#691)	43	43	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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