Retrieve the level code for use in logging.Logger.log().

def _check_loglevel(level='info'):
    '''
    Retrieve the level code for use in logging.Logger.log().
    '''
    try:
        level = level.lower()
        if level == 'quiet':
            return None
        else:
            return LOG_LEVELS[level]
    except (AttributeError, KeyError):
        log.error(
            'Invalid output_loglevel \'%s\'. Valid levels are: %s. Falling '
            'back to \'info\'.',
            level, ', '.join(sorted(LOG_LEVELS, reverse=True))
        )
        return LOG_LEVELS['info']