Return standard API errors from etcd registry by operation

Adds pkg/api/errors/etcd, which defines default conversions
for common CRUD operations from etcd to api.

pkg/api/errors/etcd/doc.go

@@ -0,0 +1,18 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+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 etcd provides conversion of etcd errors to API errors.
+package etcd