[errors] Greatly simplify common/errors package.

vpython/venv/util.go

 // Copyright 2017 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 venv
 
 import (
         "os"
 
         "github.com/golang/protobuf/proto"
 
         "github.com/luci/luci-go/common/errors"
 )
 
 func writeTextProto(path string, msg proto.Message) error {
         fd, err := os.Create(path)
         if err != nil {
-                return errors.Annotate(err).Reason("failed to create output file").Err()
+                return errors.Annotate(err, "failed to create output file").Err()
         }
 
         if err := proto.MarshalText(fd, msg); err != nil {
                 _ = fd.Close()
-                return errors.Annotate(err).Reason("failed to output text protobuf").Err()
+                return errors.Annotate(err, "failed to output text protobuf").Err()
         }
 
         if err := fd.Close(); err != nil {
-                return errors.Annotate(err).Reason("failed to Close temporary file").Err()
+                return errors.Annotate(err, "failed to Close temporary file").Err()
         }
 
         return nil
 }