[errors] Greatly simplify common/errors package.

luci_config/server/cfgclient/backend/authority.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 34 matching lines @@
 // We use a shorthand notation so that we don't waste space in JSON.
 func (a Authority) MarshalJSON() ([]byte, error) {
         switch a {
         case AsAnonymous:
                 return asAnonymousJSON, nil
         case AsService:
                 return asServiceJSON, nil
         case AsUser:
                 return asUserJSON, nil
         default:
-                return nil, errors.Reason("unknown authority: %(auth)v").D("auth", a).Err()
+                return nil, errors.Reason("unknown authority: %v", a).Err()
         }
 }
 
 // UnmarshalJSON implements encoding/json.Unmarshaler.
 func (a *Authority) UnmarshalJSON(d []byte) error {
         switch {
         case bytes.Equal(d, asAnonymousJSON):
                 *a = AsAnonymous
         case bytes.Equal(d, asServiceJSON):
                 *a = AsService
         case bytes.Equal(d, asUserJSON):
                 *a = AsUser
         default:
-                return errors.Reason("unknown authority JSON value: %(auth)v").D("auth", d).Err()
+                return errors.Reason("unknown authority JSON value: %v", d).Err()
         }
         return nil
 }