[errors] Greatly simplify common/errors package.

vpython/venv/venv_test.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 (
         "encoding/json"
         "flag"
         "fmt"
@@ skipping 249 matching lines @@
 
                 t.Run(fmt.Sprintf(`Testing Virtualenv for: %s`, tc.name), func(t *testing.T) {
                         testVirtualEnvWith(t, tc.ri)
                 })
         }
 }
 
 func loadJSON(path string, dst interface{}) error {
         content, err := ioutil.ReadFile(path)
         if err != nil {
-                return errors.Annotate(err).Reason("failed to open file").Err()
+                return errors.Annotate(err, "failed to open file").Err()
         }
         if err := json.Unmarshal(content, dst); err != nil {
-                return errors.Annotate(err).Reason("failed to unmarshal JSON").Err()
+                return errors.Annotate(err, "failed to unmarshal JSON").Err()
         }
         return nil
 }
 
 func shouldNotExist(actual interface{}, expected ...interface{}) string {
         path := actual.(string)
         switch _, err := os.Stat(path); {
         case err == nil:
                 return fmt.Sprintf("Path %q should not exist, but it does.", path)
         case os.IsNotExist(err):
                 return ""
         default:
                 return fmt.Sprintf("Couldn't check if %q exists: %s", path, err)
         }
 }