Optimize auth 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.

The issue names no files, tests, or entry points. First locate the authentication error-handling path and clarify which large-dataset workflow is affected, then define the expected descriptive messages and add coverage for the agreed cases. Done means the affected errors provide the agreed debugging context and the related tests pass.

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