[errors] Greatly simplify common/errors package.

common/tsmon/config.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 tsmon
 
 import (
         "encoding/json"
         "os"
         "runtime"
@@ skipping 19 matching lines @@
                 return ret, nil
         }
 
         file, err := os.Open(path)
         switch {
         case err == nil:
                 defer file.Close()
 
                 decoder := json.NewDecoder(file)
                 if err = decoder.Decode(&ret); err != nil {
-                        return ret, errors.Annotate(err).Reason("failed to decode file").Err()
+                        return ret, errors.Annotate(err, "failed to decode file").Err()
                 }
                 return ret, nil
 
         case os.IsNotExist(err):
                 // The file does not exist. We don't consider this an error, since the file
                 // is optional.
                 return ret, nil
 
         default:
                 // An unexpected failure occurred.
-                return ret, errors.Annotate(err).Reason("failed to open file").Err()
+                return ret, errors.Annotate(err, "failed to open file").Err()
         }
 }
 
 func defaultConfigFilePath() string {
         // TODO(vadimsh): Move this to "hardcoded/chromeinfra" package.
         if runtime.GOOS == "windows" {
                 return "C:\\chrome-infra\\ts-mon.json"
         }
         return "/etc/chrome-infra/ts-mon.json"
 }