[errors] Greatly simplify common/errors package.

luci_config/server/cfgclient/backend/get_all.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 backend
 
 import (
         "bytes"
 
         "github.com/luci/luci-go/common/errors"
@@ skipping 18 matching lines @@
 )
 
 // MarshalJSON implements json.Marshaler.
 func (gat GetAllTarget) MarshalJSON() ([]byte, error) {
         switch gat {
         case GetAllProject:
                 return projectJSON, nil
         case GetAllRef:
                 return refJSON, nil
         default:
-                return nil, errors.Reason("unknown GetAllTarget: %(value)v").D("value", gat).Err()
+                return nil, errors.Reason("unknown GetAllTarget: %v", gat).Err()
         }
 }
 
 // UnmarshalJSON implements json.Unmarshaler.
 func (gat *GetAllTarget) UnmarshalJSON(d []byte) error {
         switch {
         case bytes.Equal(d, projectJSON):
                 *gat = GetAllProject
         case bytes.Equal(d, refJSON):
                 *gat = GetAllRef
         default:
-                return errors.Reason("unknown GetAllTarget: %(value)q").D("value", d).Err()
+                return errors.Reason("unknown GetAllTarget: %q", d).Err()
         }
         return nil
 }