Add unit tests 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.

The issue does not identify handler files, tests, or an entry point. Start by locating the API handlers and their current error paths, then determine which cases need descriptive messages and corresponding unit tests; done means the intended error behavior is covered and the messages are more useful for debugging.

Written by the indexing model from the issue text.