[errors] Greatly simplify common/errors package.

deploytool/cmd/luci_deploy/param.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 (
         "regexp"
         "strings"
 
@@ skipping 22 matching lines @@
                 // v[m[2]:m[3]] is the paramter key (first capture group)
                 parts = append(parts, v[sidx:m[0]])
                 sidx = m[1]
 
                 key := v[m[2]:m[3]]
                 if val, ok := subs[key]; ok {
                         parts = append(parts, val)
                 } else {
                         // panic to immediately stop matching. This will be caught at the top of
                         // this function.
-                        return errors.Reason("undefined parameter %(key)q").D("key", key).Err()
+                        return errors.Reason("undefined parameter %q", key).Err()
                 }
         }
 
         // Finish the string and return.
         parts = append(parts, v[sidx:])
         *vp = strings.Join(parts, "")
         return nil
 }