Add unit tests for models

**Problem**
When working with components, 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 locating the models, components, existing error-handling code, and current unit-test structure. Determine which component failures need more descriptive messages, then add tests that verify the improved messages and confirm the relevant test suite passes.

Written by the indexing model from the issue text.