def download_version(servermap, version):
    """Download a specific version of the file, using the servermap
        as a guide to where the shares are located.

        I return a Deferred that fires with the requested contents, or
        errbacks with UnrecoverableFileError. Note that a servermap that was
        updated with MODE_ANYTHING or MODE_READ may not know about shares for
        all versions (those modes stop querying servers as soon as they can
        fulfil their goals), so you may want to use MODE_CHECK (which checks
        everything) to get increased visibility.
        """
