[errors] Greatly simplify common/errors package.

dm/appengine/mutate/timeout_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 mutate
 
 import (
         "fmt"
         "time"
 
@@ skipping 69 matching lines @@
                 if err != nil {
                         logging.Fields{
                                 logging.ErrorKey: err,
                                 "cfgName":        e.DistributorConfigName,
                         }.Errorf(c, "Could not MakeDistributor")
                         return
                 }
                 var realRslt *dm.Result
                 q := model.QuestFromID(t.For.Quest)
                 if err = ds.Get(ds.WithoutTransaction(c), q); err != nil {
-                        err = errors.Annotate(err).Reason("loading quest").Err()
+                        err = errors.Annotate(err, "loading quest").Err()
                         return
                 }
                 realRslt, err = dist.GetStatus(&q.Desc, distributor.Token(e.DistributorToken))
                 if (err != nil || realRslt == nil) && t.TimeoutAttempt < maxTimeoutAttempts {
                         logging.Fields{
                                 logging.ErrorKey:  err,
                                 "task_result":     realRslt,
                                 "timeout_attempt": t.TimeoutAttempt,
                         }.Infof(c, "GetStatus failed/nop'd while timing out STOPPING execution")
                         // TODO(riannucci): do randomized exponential backoff instead of constant
@@ skipping 46 matching lines @@
                 return tumble.CancelNamedMutations(c, key, "timeout")
         }
         return tumble.PutNamedMutations(c, key, map[string]tumble.Mutation{
                 "timeout": &TimeoutExecution{eid, e.State, 0, clock.Now(c).UTC().Add(howLong)},
         })
 }
 
 func init() {
         tumble.Register((*TimeoutExecution)(nil))
 }