Improve error handling for GetCredentialStateForProvider

[dmjb]

The GetCredentialStateForProvider swallows an error since it was originally written for a codepath where the error was not important. Since then, it is used in a second place where the error is important. I'm not sure if it's possible for the error to get triggered in practice, but I managed to trigger the error path while writing unit tests, which led to a strange and hard to debug error happening after the error was skipped over.

Refactor this function to return an error. Ignore it in the call site where it should be ignored, and react to it where it is significant. As a nice side effect of this change, the control flow of GetCredentialStateForProvider is significantly simplified.

Some unrelated unit test improvements from an ongoing development branch have also been added to this branch.

Summary

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Fixes #(related issue)

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• Bug fix (resolves an issue without affecting existing features)
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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
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[coveralls]

coverage: 50.215% (-0.06%) from 50.276%
when pulling ac1165d on return-error-cred-state
into 006c6fa on main.