[errors] Greatly simplify common/errors package.

vpython/venv/system_posix.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.
 
 // +build darwin linux freebsd netbsd openbsd android
 
 package venv
 
 import (
         "os"
@@ skipping 10 matching lines @@
 }
 
 // venvBinDir resolves the path where VirtualEnv binaries are installed.
 func venvBinDir(root string) string {
         return filepath.Join(root, "bin")
 }
 
 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
 }