Prompt GCP authentication flow about bigquery-etl

[sean-rose]

Currently, users that are not logged in will see an error message with instructions to perform the authentication themselves.

Currently users will only get that error message if they have no Google application default credentials at all. Once they've set application default credentials the first time they will no longer get that error message, even if their credentials have expired and they need to re-authenticate. I think different authentication detection logic is needed to handle expired credentials, as suggested in #5171.

The tricky part here is that the authentication flow also needs to account for non-interactive environments where no browser is available (testing env, Airflow). We do not want runs in these environments to get stuck, so we need a way to exit runs if no credentials are available.

Perhaps we could check for standard environment variables always set by CircleCI, Airflow, and/or GKE?