Add unit tests for lib

**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.

Start by locating the lib code for store operations and the existing unit-test setup. Read how current errors are produced, then define tests for the requested descriptive error messages and run the relevant lib test suite. Done means store failures expose the added detail and the unit tests pass.

Written by the indexing model from the issue text.