[errors] de-specialize Transient in favor of Tags.

common/retry/wrap.go

-// Copyright 2015 The LUCI Authors. All rights reserved.
-// Use of this source code is governed under the Apache License, Version 2.0
-// that can be found in the LICENSE file.
-
-package retry
-
-// wrap wraps a Factory, applying a modification function to each Iterator
-// that it produces.
-func wrap(g Factory, mod func(it Iterator) Iterator) Factory {
-        return func() Iterator {
-                next := g()
-                if next == nil {
-                        return nil
-                }
-                return mod(next)
-        }
-}