Fix #142: improve error message when parsing book.json
Fix #142: improve error message when parsing book.json

zhanghuiguoanlige/editor-legacy,zhanghuiguoanlige/editor-legacy,gdbooks/editor-legacy,gdbooks/editor-legacy,gdbooks/editor-legacy,KINGSABRI/editor-legacy,GitbookIO/editor-legacy,Futuring/editor-legacy,Futuring/editor-legacy,Futuring/editor-legacy,GitbookIO/editor-legacy,zhanghuiguoanlige/editor-legacy,KINGSABRI/editor-legacy,KINGSABRI/editor-legacy,GitbookIO/editor-legacy