Add TypeScript types for handlers

**Problem**
When working with api, 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. First reconcile the title's TypeScript handler types with the body’s request for more descriptive API errors, then locate the handler definitions and existing error tests; done means the agreed scope is implemented and the relevant tests pass.

Written by the indexing model from the issue text.