Improve error messages in 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 names no files, tests, or entry points. Start by locating the store and model error-handling code, then identify which failures currently lack useful context. Done means the relevant errors have more descriptive messages and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.