Do executes the "compute.regionInstanceGroups.listInstances" call. Exactly one of *RegionInstanceGroupsListInstances or error will be non-nil. Any non-2xx status code is an error. Response headers are in either *RegionInstanceGroupsListInstances.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.