[errors] Greatly simplify common/errors package.

vpython/venv/system_windows.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"
         "path/filepath"
         "syscall"
 
         "github.com/luci/luci-go/common/errors"
 )
 
 // longestGeneratedScriptPath returns the path of the longest generated script
 // given a VirtualEnv root.
 //
 // Since Windows doesn't generate scripts, this is disabled.
 func longestGeneratedScriptPath(basedir string) string { return "" }
 
 // venvBinDir resolves the path where VirtualEnv binaries are installed.
 func venvBinDir(root string) string {
         return filepath.Join(root, "Scripts")
 }
 
 func checkProcessRunning(pid int) error {
         proc, err := os.FindProcess(pid)
         if err != nil {
-                return errors.Annotate(err).Reason("failed to find process").Err()
+                return errors.Annotate(err, "failed to find process").Err()
         }
 
         if err := proc.Signal(os.Signal(syscall.Signal(0))); err != nil {
-                return errors.Annotate(err).Reason("failed to signal process").Err()
+                return errors.Annotate(err, "failed to signal process").Err()
         }
         return nil
 }