Optimize handlers for large datasets

**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, tests, or entry points are named. Start by locating the authentication handlers and their existing error paths; done means the relevant authentication failures produce more descriptive messages and the associated behavior is covered by tests.

Written by the indexing model from the issue text.