Update lib 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.

The issue only points to lib documentation and middleware error handling, so first locate the middleware entry point and its current error messages under lib. Review the existing behavior and documentation, then clarify which messages and documented cases should change; done means the requested middleware failures are described more clearly and the documentation matches the behavior.

Written by the indexing model from the issue text.