Fix error message from KeyForObj when passing an invalid struct.

service/datastore/multiarg.go

Index: service/datastore/multiarg.go
diff --git a/service/datastore/multiarg.go b/service/datastore/multiarg.go
index cc1a6726510f6817887f6539a42ba276c780e78b..ad141d3db7bd4c88becbdeb6682d89531df7bc53 100644
--- a/service/datastore/multiarg.go
+++ b/service/datastore/multiarg.go
@@ -258,23 +258,45 @@ func multiArgTypeInterface() multiArgType {
 
 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
 }