Add unit tests for api

**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 file, entry point, or existing test path is named. Start by locating the API error-handling code and current unit tests, then determine the expected descriptive messages; done means the error behavior is improved and covered by tests.

Written by the indexing model from the issue text.