fix: don't retry if error or error_description is not string

[arithmetic1728]

If users forget to set quota project id in user credential ADC file, and reauth is needed, the server returns the following error:

{'error': 
    {'code': 403, 
      'message': "Your application has authenticated using end user credentials from the Google Cloud SDK or Google Cloud Shell which are not supported by the reauth.googleapis.com. We recommend configuring the billing/quota_project setting in gcloud or using a service account through the auth/impersonate_service_account setting..."
    }
}

Note that the 'error' value is not a string, so the if any(e in retryable_error_descriptions for e in (error_code, error_desc)) line throws an exception saying unhashable type: 'dict'. We should simply return if either error or error_description is not a string.

Internal bug: b/270627288

This PR also fixes a flaky sample test by setting a 10 seconds clock skew when verifying id token.