Improve error messages in middleware

**Problem**
When working with validation, 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 files, tests, or entry points are named. Start by locating the middleware validation and error-handling paths, then inspect existing error-message tests or add focused coverage for the expected descriptive output. Done means the validation failures provide the agreed debugging context without breaking current behavior.

Written by the indexing model from the issue text.