Checks to see if the wkhtmltohtml binary is installed. Raises error if not.

def check_for_wkhtmltohtml():
    """
    Checks to see if the wkhtmltohtml binary is installed. Raises error if not.
    """
    locator = 'where' if sys.platform == 'win32' else 'which'

    wkhtmltopdf = (subprocess.Popen([locator, 'wkhtmltopdf'],
                                    stdout=subprocess.PIPE)
                   .communicate()[0].strip())

    if not os.path.exists(wkhtmltopdf):
        logging.error(
            'No wkhtmltopdf executable found. Please install '
            'wkhtmltopdf before trying again - {}'.format(WKHTMLTOPDF_URL))
        raise ValueError(
            'No wkhtmltopdf executable found. Please install '
            'wkhtmltopdf before trying again - {}'.format(WKHTMLTOPDF_URL))