[errors] Greatly simplify common/errors package.

logdog/common/storage/entry.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 storage
 
 import (
         "github.com/luci/luci-go/common/errors"
         "github.com/luci/luci-go/logdog/api/logpb"
         "github.com/luci/luci-go/logdog/common/types"
@@ skipping 50 matching lines @@
 // underlying data.
 func (e *Entry) GetLogEntry() (*logpb.LogEntry, error) {
         if e.le == nil {
                 if e.D == nil {
                         // This can happen with keys-only results.
                         return nil, errors.New("no log entry data")
                 }
 
                 var le logpb.LogEntry
                 if err := proto.Unmarshal(e.D, &le); err != nil {
-                        return nil, errors.Annotate(err).Reason("failed to unmarshal").Err()
+                        return nil, errors.Annotate(err, "failed to unmarshal").Err()
                 }
                 e.le = &le
         }
 
         return e.le, nil
 }