Refactor middleware for better readability

**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 and validation error-handling code, then inspect how current messages are produced. Done means the middleware is easier to read and validation failures provide more descriptive messages, with any relevant existing checks still passing.

Written by the indexing model from the issue text.