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 handlers and the data structures involved in their error paths. Read the existing error handling to determine which messages are currently available and where TypeScript types are missing. Done should mean the requested data types exist and handler failures provide more descriptive error messages, but the issue does not name specific files or tests.

Written by the indexing model from the issue text.