Consider deprecating old lib API

**Problem**
When working with validation, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

No files or tests are named. Start by locating the validation logic and the old lib API, then determine which API is being deprecated and where its errors are produced; done means the deprecation path and more descriptive validation errors are defined and covered by the relevant checks.

Written by the indexing model from the issue text.