Move multierror logic into luci-go.

common/errors/doc.go

Index: common/errors/doc.go
diff --git a/common/errors/doc.go b/common/errors/doc.go
new file mode 100644
index 0000000000000000000000000000000000000000..8b3a02aa6ab0a3597706fbe9183cdde55c8083fd
--- /dev/null
+++ b/common/errors/doc.go
@@ -0,0 +1,8 @@
+// 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 errors is an augmented replacement package for the stdlib "errors"
+// package. It contains the same New method, but also has some handy methods
+// and types for dealing with errors.
+package errors