Add unit tests for store

**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 does not name a file, test, or store entry point. First locate the store implementation and its existing unit-test entry point, then clarify which model error cases need coverage; done means the requested cases have tests and the resulting errors are more descriptive.

Written by the indexing model from the issue text.