Read and return `DM_SQUARE_DEBUG` env var, if defined.

    Raises
    ------
    RuntimeError
        If DM_SQUARE_DEBUG is not an int convertable value

def debug_lvl_from_env():
    """Read and return `DM_SQUARE_DEBUG` env var, if defined.

    Raises
    ------
    RuntimeError
        If DM_SQUARE_DEBUG is not an int convertable value
    """
    debug_lvl = os.environ.get('DM_SQUARE_DEBUG')
    if not debug_lvl:
        return 0

    try:
        debug_lvl = int(debug_lvl)
    except ValueError:
        # ensure that logging is configured as this method is likely to be
        # called prior to configuring logging.
        setup_logging(verbosity=1)
        raise RuntimeError(
            textwrap.dedent("""\
            env var DM_SQUARE_DEBUG '{debug_lvl}' is not a string value that
            can be converted to an int.""".format(debug_lvl=debug_lvl))
        ) from None