[errors] Greatly simplify common/errors package.

dm/appengine/mutate/finish_attempt.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 mutate
 
 import (
         "google.golang.org/grpc/codes"
 
         ds "github.com/luci/gae/service/datastore"
@@ skipping 37 matching lines @@
 
         ar := &model.AttemptResult{
                 Attempt: model.AttemptKeyFromID(c, &atmpt.ID),
                 Data:    *f.Data,
         }
 
         rslt := *f.Data
         atmpt.Result.Data = &rslt
         atmpt.Result.Data.Object = ""
 
-        err = errors.Annotate(ds.Put(c, atmpt, ar)).Tag(grpcutil.Tag.With(codes.Internal)).
-                Reason("during Put").Err()
+        err = errors.Annotate(ds.Put(c, atmpt, ar), "during Put").Tag(grpcutil.Tag.With(codes.Internal)).Err()
         return
 }
 
 func init() {
         tumble.Register((*FinishAttempt)(nil))
 }