Invalid cbauth credential responses lead to failed queries

When using the dynamic authentication mechanism implemented for the eventing service, there is no way to signal that credentials are not available. This can lead to operations being rejected due to the request being invalid if there are no credentials.

This issue is primarily to track refactoring of the interface provided to the eventing service to enable error's to be reported from the credential provider (which may already be possible through the Authenticator mechanism provided by the SDK).