Add unit tests for lib

**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.

The issue only identifies the lib area and configuration-related error handling; start by locating the relevant configuration code and the repository's existing unit-test setup. Define the error cases and expected messages before changing them, then add coverage for those cases and verify that the test suite passes.

Written by the indexing model from the issue text.