Add new metadata store.

Signed-off-by: Lantao Liu <lantaol@google.com>

pkg/store/errors.go

@@ -0,0 +1,27 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package store
+
+import "errors"
+
+var (
+	// ErrAlreadyExist is the error returned when data added in the store
+	// already exists.
+	ErrAlreadyExist = errors.New("already exists")
+	// ErrNotExist is the error returned when data is not in the store.
+	ErrNotExist = errors.New("does not exist")
+)