Consider deprecating old lib API

**Problem**
When working with config, 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 configuration error-handling code and the old lib API referenced in the title, then clarify whether the goal is deprecation, more descriptive messages, or both; done requires an agreed scope and corresponding tests or documented behavior.

Written by the indexing model from the issue text.