[errors] Greatly simplify common/errors package.

vpython/python/python.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 python
 
 import (
         "strings"
         "unicode/utf8"
 
@@ skipping 86 matching lines @@
                         // "-" instructs Python to load the script from STDIN.
                         cmd.Target = ScriptTarget{
                                 Path: "-",
                         }
                         break
                 }
 
                 // Extract the flag Target. -f<lag>
                 r, l := utf8.DecodeRuneInString(flag)
                 if r == utf8.RuneError {
-                        err = errors.Reason("invalid rune in flag #%(index)d").D("index", i).Err()
+                        err = errors.Reason("invalid rune in flag #%d", i).Err()
                         return
                 }
 
                 // Is this a combined flag/value (e.g., -c'paoskdpo') ?
                 flag = flag[l:]
                 twoVarType := func() (string, error) {
                         if len(flag) > 0 {
                                 return flag, nil
                         }
 
                         if i >= len(args) {
-                                return "", errors.Reason("two-value flag -%(flag)c missing second value at %(index)d").
-                                        D("flag", r).
-                                        D("index", i).
-                                        Err()
+                                return "", errors.Reason("two-value flag -%c missing second value at %d", r, i).Err()
                         }
 
                         value := args[i]
                         i++
                         return value, nil
                 }
 
                 switch r {
                 // Two-variable execution flags.
                 case 'c':
@@ skipping 38 matching lines @@
         cmd.Args = args[i:]
         return
 }
 
 func trimPrefix(v, pfx string) (string, bool) {
         if strings.HasPrefix(v, pfx) {
                 return v[len(pfx):], true
         }
         return v, false
 }