[errors] Greatly simplify common/errors package.

deploytool/cmd/luci_deploy/title.go

 // Copyright 2016 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 main
 
 import (
         "path/filepath"
         "unicode"
 
         "github.com/luci/luci-go/common/errors"
 )
 
 type title string
 
 func (t title) validate() error {
         if len(t) == 0 {
                 return errors.New("cannot be empty")
         }
 
         idx := 0
         for _, r := range t {
                 if !(unicode.IsLetter(r) || unicode.IsNumber(r) || r == '-') {
-                        return errors.Reason("character at %(pos)d (%(char)c) is not permitted in a title").
-                                D("pos", idx).D("char", r).Err()
+                        return errors.Reason("character at %d (%c) is not permitted in a title", idx, r).Err()
                 }
                 idx++
         }
         return nil
 }
 
 // titleFromConfigPath returns the title of a configuration item identified by
 // the specified configuration file.
 //
 // If the file was not a valid config path, or the title was not valid, an error
 // will be returned.
 func titleFromConfigPath(path string) (title, error) {
         path = filepath.Base(path)
         if filepath.Ext(path) == configExt {
                 t := title(path[:len(path)-len(configExt)])
                 if err := t.validate(); err != nil {
                         return "", err
                 }
                 return t, nil
         }
         return "", errors.Reason("missing config extension [" + configExt + "]").Err()
 }