[errors] Greatly simplify common/errors package.

deploytool/cmd/luci_deploy/path.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 (
         "io/ioutil"
         "os"
         "path/filepath"
@@ skipping 40 matching lines @@
         if filepath.Separator == '/' {
                 return path
         }
         return strings.Replace(path, "/", string(filepath.Separator), -1)
 }
 
 func withTempDir(f func(string) error) error {
         // Create a temporary directory.
         tdir, err := ioutil.TempDir("", "luci_deploytool")
         if err != nil {
-                return errors.Annotate(err).Reason("failed to create tempdir").Err()
+                return errors.Annotate(err, "failed to create tempdir").Err()
         }
         defer os.RemoveAll(tdir)
 
         return f(tdir)
 }