Add unit tests for core

**Problem**
When working with lib, 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 lib directory and locating the core error-handling entry points and existing unit-test setup. Determine which errors need more detail and document the expected messages before making changes; done means the core behavior has focused unit tests and the resulting errors are demonstrably more descriptive.

Written by the indexing model from the issue text.