improve export error message
improve export error message

koba-e964/picrin,ktakashi/picrin,dcurrie/picrin,leavesbnw/picrin,picrin-scheme/picrin,omasanori/picrin,picrin-scheme/picrin,leavesbnw/picrin,leavesbnw/picrin,ktakashi/picrin,ktakashi/picrin,dcurrie/picrin,omasanori/picrin,koba-e964/picrin,koba-e964/picrin