Fix error message from KeyForObj when passing an invalid struct.

service/datastore/errors.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package datastore
 
 import (
         "fmt"
         "reflect"
 
         "google.golang.org/appengine/datastore"
 )
 
 // These errors are returned by various datastore.Interface methods.
 var (
-        ErrInvalidEntityType     = datastore.ErrInvalidEntityType
         ErrInvalidKey            = datastore.ErrInvalidKey
         ErrNoSuchEntity          = datastore.ErrNoSuchEntity
         ErrConcurrentTransaction = datastore.ErrConcurrentTransaction
-
-        // ErrMetaFieldUnset is returned from PropertyLoadSaver.{Get,Set}Meta
-        // implementations when the specified meta key isn't set on the struct at
-        // all.
-        ErrMetaFieldUnset = fmt.Errorf("gae: meta field unset")
 )
 
 // ErrFieldMismatch is returned when a field is to be loaded into a different
 // type than the one it was stored from, or when a field is missing or
 // unexported in the destination struct.
 // StructType is the type of the struct pointed to by the destination argument
 // passed to Get or to Iterator.Next.
 type ErrFieldMismatch struct {
         StructType reflect.Type
         FieldName  string
         Reason     string
 }
 
 func (e *ErrFieldMismatch) Error() string {
         return fmt.Sprintf("gae: cannot load field %q into a %q: %s",
                 e.FieldName, e.StructType, e.Reason)
 }