Update store documentation

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

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

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

No file, test, or entry point is named. Start by locating the store documentation and middleware error-handling code, then clarify whether the goal is documentation changes, more descriptive runtime errors, or both; done means the agreed scope is documented and any specified error-message behavior is covered.

Written by the indexing model from the issue text.