Do executes the "identitytoolkit.relyingparty.verifyCustomToken" call. Exactly one of *VerifyCustomTokenResponse or error will be non-nil. Any non-2xx status code is an error. Response headers are in either *VerifyCustomTokenResponse.ServerResponse.Header or (if a response was returned at all) in error.(*googleapi.Error).Header. Use googleapi.IsNotModified to check whether the returned error was because http.StatusNotModified was returned.