[errors] de-specialize Transient in favor of Tags.

scheduler/appengine/task/utils/apiclient.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package utils
 
 import (
-        "github.com/luci/luci-go/common/errors"
+        "github.com/luci/luci-go/common/retry"
         "google.golang.org/api/googleapi"
 )
 
 // IsTransientAPIError returns true if error from Google API client indicates
 // an error that can go away on its own in the future.
 func IsTransientAPIError(err error) bool {
         if err == nil {
                 return false
         }
         apiErr, _ := err.(*googleapi.Error)
         if apiErr == nil {
                 return true // failed to get HTTP code => connectivity error => transient
         }
         return apiErr.Code >= 500 || apiErr.Code == 429 || apiErr.Code == 0
 }
 
 // WrapAPIError wraps error from Google API client in transient wrapper if
 // necessary.
 func WrapAPIError(err error) error {
         if IsTransientAPIError(err) {
-                return errors.WrapTransient(err)
+                return retry.Tag.Apply(err)
         }
         return err
 }