[errors] Greatly simplify common/errors package.

appengine/gaeauth/server/oauth.go

 // Copyright 2015 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 server
 
 import (
         "fmt"
         "net/http"
         "strings"
@@ skipping 92 matching lines @@
         logging.Infof(c, "oauth: Querying tokeninfo endpoint")
         tokenInfo, err := googleoauth.GetTokenInfo(c, googleoauth.TokenInfoParams{
                 AccessToken: accessToken,
                 Client:      &http.Client{Transport: urlfetch.Get(c)},
                 Endpoint:    m.tokenInfoEndpoint,
         })
         if err != nil {
                 if err == googleoauth.ErrBadToken {
                         return nil, err
                 }
-                return nil, errors.Annotate(err).Reason("oauth: transient error when validating token").
+                return nil, errors.Annotate(err, "oauth: transient error when validating token").
                         Tag(transient.Tag).Err()
         }
 
         // Verify the token contains a validated email.
         switch {
         case tokenInfo.Email == "":
                 return nil, fmt.Errorf("oauth: token is not associated with an email")
         case !tokenInfo.EmailVerified:
                 return nil, fmt.Errorf("oauth: email %s is not verified", tokenInfo.Email)
         }
@@ skipping 18 matching lines @@
                 return nil, err
         }
         u := &auth.User{
                 Identity: id,
                 Email:    tokenInfo.Email,
                 ClientID: tokenInfo.Aud,
         }
         proccache.Put(c, tokenCheckCache(accessToken), u, time.Duration(tokenInfo.ExpiresIn)*time.Second)
         return u, nil
 }