[errors] Greatly simplify common/errors package.

vpython/venv/iterator.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 (
         "io"
         "os"
         "strings"
@@ skipping 57 matching lines @@
                 }
 
                 return nil
         })
 }
 
 func iterDir(c context.Context, dirPath string, cb func([]os.FileInfo) error) error {
         // Get a listing of all VirtualEnv within the base directory.
         dir, err := os.Open(dirPath)
         if err != nil {
-                return errors.Annotate(err).Reason("failed to open base directory: %(dir)s").
-                        D("dir", dirPath).
-                        Err()
+                return errors.Annotate(err, "failed to open base directory: %s", dirPath).Err()
         }
         defer dir.Close()
 
         for done := false; !done; {
                 // Check if we've been cancelled.
                 select {
                 case <-c.Done():
                         return c.Err()
                 default:
                 }
 
                 // Read the next batch of directories.
                 fileInfos, err := dir.Readdir(forEachReadDirSize)
                 switch err {
                 case nil:
 
                 case io.EOF:
                         done = true
 
                 default:
-                        return errors.Annotate(err).Reason("could not read directory contents: %(dir)s").
-                                D("dir", dirPath).
-                                Err()
+                        return errors.Annotate(err, "could not read directory contents: %s", dirPath).Err()
                 }
 
                 if err := cb(fileInfos); err != nil {
                         return err
                 }
         }
 
         return nil
 }