Update middleware documentation

**Problem**
When working with handlers, 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 middleware documentation and the handler error paths referenced by the issue. First compare the current documentation with the existing error messages, then determine the intended scope with a maintainer. Done means the middleware guidance and handler errors are more descriptive and the relevant checks pass.

Written by the indexing model from the issue text.