def duns_screener(duns):
    """
    Takes a duns number and returns a modified string to comply with DUNS+4 format
    
    common DUNS errors:
   
    * leading zero removed: len == 8 --> add leading zero back + '0000' trailing
    * 9-digits --> duns + '0000'
    * else: 'error'
    
    """
    
    if len(duns) == 9:
        duns = duns + '0000'
        return duns
    elif len(duns) == 8:
        duns = '0' + duns + '0000'
        return duns
    else:
        return 'error'