Add unit tests for core

**Problem**
When working with store, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

Locate the store and core implementation, then inspect the existing unit-test setup and current error paths. Reproduce the existing store errors and determine which messages need more detail; done means focused tests cover the agreed behavior and the relevant test suite passes.

Written by the indexing model from the issue text.