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.

Start by locating the old configuration API and the authentication error-handling entry points; the issue names no files or tests. Clarify the deprecation scope, the replacement path, and which detailed error messages are required before making changes; done should include agreed behavior and coverage for the affected errors.

Written by the indexing model from the issue text.