Add unit tests for core

**Problem**
When working with models, 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 names no files, tests, or entry points; first locate the core model error-handling code and existing unit-test setup. Confirm which error cases and message details are required, then add tests and improve messages so model failures are more descriptive.

Written by the indexing model from the issue text.