Add unit tests for lib

**Problem**
When working with data, 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 examining the lib directory and locating the existing error-handling paths and any current unit-test conventions. The issue does not name specific files, error cases, or a test command; done should mean the relevant error cases have tests and their messages provide more debugging detail.

Written by the indexing model from the issue text.