Add TypeScript types for types

**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 the existing error-handling paths. Clarify whether the intended scope is adding TypeScript types, improving error messages, or both, then identify the affected entry points and tests. Done should mean the agreed errors are more descriptive and the requested types are covered by tests.

Written by the indexing model from the issue text.