Fail job explicitly with error message if `identity: google_cloud` is present and the prerequisites are not fulfilled

Currently, if a job contains identity: google_cloud and the prerequisites are not fulfilled, it will just be ignored and the CLOUDSDK_AUTH_CREDENTIAL_FILE_OVERRIDE and GOOGLE_APPLICATION_CREDENTIALS variables will not get defined, causing errors later on when trying to connect to Google Cloud. A better approach would be to explicitly call out the missing prerequisites in the job log.