Refactor a bit.

service/memcache/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.
+
+// This file contains errors which are mirrors/duplicates of the
+// upstream SDK errors. This exists so that users can depend solely on this
+// wrapper library without also needing to import the SDK.
+
+package memcache
+
+import (
+        orig "google.golang.org/appengine/memcache"
+)
+
+// These are pass-through versions from the managed-VM SDK. All implementations
+// must return these (exact) errors (not just an error with the same text).
+var (
+        ErrCacheMiss   = orig.ErrCacheMiss
+        ErrCASConflict = orig.ErrCASConflict
+        ErrNoStats     = orig.ErrNoStats
+        ErrNotStored   = orig.ErrNotStored
+        ErrServerError = orig.ErrServerError
+)