Add TypeScript types for data

**Problem**
When working with handlers, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

Start by locating the handler entry points and the data passed through them; no specific files or tests are named in the issue. Clarify which data needs TypeScript types and which handler errors need more detail, then define completion as typed data and descriptive error messages covered by appropriate tests.

Written by the indexing model from the issue text.