Currently, if I happen to have GOOGLE_APPLICATION_CREDENTIALS pointing to a file with user creds, loading those credentials will (rightly!) fail to determine an associated project.

However, we then get a log message suggesting that we should set an environment variable that's never checked in this case. This can be annoying for users.

Checking os.environ[environment_vars.PROJECT] is a one-liner, and seems like it'd fit with the intention in that code. I'm happy to send a PR if that's 👍.