Improve error messages in middleware

**Problem**
When working with lib, 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 points generally to middleware in lib, but names no specific file, error path, or test. Start by locating the middleware implementation and reading its existing error handling; clarify which messages need improvement and how they should be validated before changing anything. Done means the requested error cases produce detailed, useful messages with appropriate tests.

Written by the indexing model from the issue text.