[errors] Greatly simplify common/errors package.

deploytool/cmd/luci_deploy/config.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 main
 
 import (
         "path/filepath"
 
         "github.com/luci/luci-go/common/errors"
@@ skipping 15 matching lines @@
 
         configPath := filepath.Join(path, userCfgName)
         switch err := unmarshalTextProtobuf(configPath, cfg); {
         case err == nil:
                 log.Infof(c, "Loaded user config from: %s", configPath)
 
         case isNotExist(err):
                 log.Debugf(c, "No user config found at: %s", configPath)
 
         default:
-                return errors.Annotate(err).Reason("failed to load config at [%(path)s]").D("path", configPath).Err()
+                return errors.Annotate(err, "failed to load config at [%s]", configPath).Err()
         }
 
         return nil
 }