The `(dockerPusher).Push` method uses a `StatusTracker` to check if an
upload already happened, before repeating the upload. However, there is
no provision for failure handling. If a PUT request returns an error,
the `StatusTracker` will still see the upload as if it happened
successfully. Add a status boolean so that only successful uploads
short-circuit `Push`.
Signed-off-by: Aaron Lehmann <alehmann@netflix.com>