Add unit tests for auth

**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 or test path is named. Start by locating the authentication and API error-handling entry points and existing unit-test conventions; done means covering the relevant auth failures and producing more descriptive error messages, verified by the test suite.

Written by the indexing model from the issue text.