Refactor models for better readability

**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.

No files or tests are named. Start by locating the authentication models and their current error-handling paths, then review how existing errors are tested or surfaced. Done means the relevant authentication failures have more descriptive messages and the handling remains clear for debugging and maintenance.

Written by the indexing model from the issue text.