[errors] Greatly simplify common/errors package.

dm/appengine/distributor/notify_execution.go

 // Copyright 2016 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 distributor
 
 import (
         ds "github.com/luci/gae/service/datastore"
         "github.com/luci/luci-go/common/errors"
         "github.com/luci/luci-go/common/logging"
@@ skipping 22 matching lines @@
         if err != nil {
                 logging.Fields{
                         logging.ErrorKey: err,
                         "cfg":            f.CfgName,
                 }.Errorf(c, "Failed to make distributor")
                 return
         }
 
         q := &model.Quest{ID: f.Notification.ID.Quest}
         if err := ds.Get(ds.WithoutTransaction(c), q); err != nil {
-                return nil, errors.Annotate(err).Reason("getting Quest").Err()
+                return nil, errors.Annotate(err, "getting Quest").Err()
         }
         rslt, err := dist.HandleNotification(&q.Desc, f.Notification)
         if err != nil {
                 // TODO(riannucci): check for transient/non-transient
                 logging.Fields{
                         logging.ErrorKey: err,
                         "cfg":            f.CfgName,
                 }.Errorf(c, "Failed to handle notification")
                 return
         }
         if rslt != nil {
                 return reg.FinishExecution(c, f.Notification.ID, rslt)
         }
         return
 }
 
 func init() {
         tumble.Register((*NotifyExecution)(nil))
 }