Add unit tests for api

**Problem**
When working with auth, 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 names no files or existing tests. Start by locating the authentication API error-handling paths and their current tests. Done means the relevant failures produce more descriptive messages and unit tests cover the changed behavior.

Written by the indexing model from the issue text.