Improve error messages in lib

**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.

Start by inspecting the validation-related code under lib. The issue names no specific entry point or test, so locate existing validation coverage before changing anything. Done means validation failures have more descriptive messages and the relevant tests pass.

Written by the indexing model from the issue text.