Consider deprecating old config 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.

No files, tests, or entry points are named. Start by locating the authentication error handling and the old config API, then determine the agreed deprecation scope and the expected error-message changes before defining what completion means.

Written by the indexing model from the issue text.