Poor error message when auth is not initialized

[coryan]

Running without any auth available generates errors like these:

env GOOGLE_CLOUD_PROJECT=${P_ID} GOOGLE_CLOUD_SWIFT_TEST_SERVICE_ACCOUNT=swift-sdk-test@${P_ID}.iam.gserviceaccount.com swift test --traits IntegrationTests

### logging() error=unimplemented
✘ Test longRunningOperations() recorded an issue at Driver.swift:35:6: Caught error: unimplemented

That is a very poor error message, we should do better.