Add TypeScript types for handlers

**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 names handlers, middleware, TypeScript types, and error handling but does not identify files, tests, or an entry point. First locate the handler and middleware definitions, then clarify whether the expected result is typed handlers, more descriptive errors, or both; done should be verifiable through the repository's relevant tests.

Written by the indexing model from the issue text.