Fix error message from KeyForObj when passing an invalid struct.

service/datastore/multiarg.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"
 
@@ skipping 240 matching lines @@
                         return pls.Load(pm)
                 },
                 setKey: func(slot reflect.Value, k *Key) {
                         setKey(slot.Elem().Interface(), k)
                 },
         }
 }
 
 func newKeyObjErr(aid, ns string, src interface{}) (*Key, error) {
         pls := getMGS(src)
-        if key, _ := pls.GetMetaDefault("key", nil).(*Key); key != nil {
+        keyI, err := GetMetaDefault(pls, "key", nil)
+        if err != nil {
+                return nil, err
+        }
+        if key, _ := keyI.(*Key); key != nil {
                 return key, nil
         }
 
         // get kind
-        kind := pls.GetMetaDefault("kind", "").(string)
+        kindI, err := GetMetaDefault(pls, "kind", "")
+        if err != nil {
+                return nil, err
+        }
+
+        kind, _ := kindI.(string)
         if kind == "" {
                 return nil, fmt.Errorf("unable to extract $kind from %T", src)
         }
 
         // get id - allow both to be default for default keys
-        sid := pls.GetMetaDefault("id", "").(string)
-        iid := pls.GetMetaDefault("id", 0).(int64)
+        sidI, err := GetMetaDefault(pls, "id", "")
+        if err != nil {
+                return nil, err
+        }
+
+        iidI, err := GetMetaDefault(pls, "id", 0)
+        if err != nil {
+                return nil, err
+        }
 
         // get parent
-        par, _ := pls.GetMetaDefault("parent", nil).(*Key)
+        parI, err := GetMetaDefault(pls, "parent", nil)
+        if err != nil {
+                return nil, err
+        }
 
+        iid, _ := iidI.(int64)
+        sid, _ := sidI.(string)
+        par, _ := parI.(*Key)
         return NewKey(aid, ns, kind, sid, iid, par), nil
 }
 
 func setKey(src interface{}, key *Key) {
         pls := getMGS(src)
         if pls.SetMeta("key", key) == ErrMetaFieldUnset {
                 lst := key.LastTok()
                 if lst.StringID != "" {
                         _ = pls.SetMeta("id", lst.StringID)
                 } else {
                         _ = pls.SetMeta("id", lst.IntID)
                 }
                 _ = pls.SetMeta("kind", lst.Kind)
                 _ = pls.SetMeta("parent", key.Parent())
         }
 }
 
 func mkPLS(o interface{}) PropertyLoadSaver {
         if pls, ok := o.(PropertyLoadSaver); ok {
                 return pls
         }
         return GetPLS(o)
 }