Add unit tests for data

**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 mentions authentication error messages but names no files, tests, or entry points. Start by locating the authentication error-handling path and existing unit-test setup; done means the relevant auth failures have more descriptive messages and are covered by unit tests.

Written by the indexing model from the issue text.