[errors] Greatly simplify common/errors package.

common/config/impl/filesystem/paths.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 filesystem
 
 import (
         "io/ioutil"
         "os"
         "path"
@@ skipping 39 matching lines @@
 func (c configSet) hasPrefix(prefix luciPath) bool {
         return strings.HasPrefix(c.s(), prefix.s())
 }
 
 func (c configSet) id() string {
         return strings.Split(c.s(), "/")[1]
 }
 
 func (c configSet) validate() error {
         if !c.hasPrefix("projects/") && !c.hasPrefix("services/") {
-                return errors.Reason("configSet.validate: bad prefix %(cs)q").D("cs", c.s()).Err()
+                return errors.Reason("configSet.validate: bad prefix %q", c.s()).Err()
         }
         return nil
 }
 
 type nativePath string
 
 func (n nativePath) explode() []string {
         return strings.Split(n.s(), string(filepath.Separator))
 }
 
@@ skipping 14 matching lines @@
         return ioutil.ReadFile(n.s())
 }
 
 func (n nativePath) toLUCI() luciPath {
         return luciPath(filepath.ToSlash(n.s()))
 }
 
 func (n nativePath) s() string {
         return string(n)
 }