more user-friendly error message for people who forget to specify history_dir
more user-friendly error message for people who forget to specify history_dir

BBC-News/wraith,saulirajala/wraith,adouglas/wraith,peruukki/wraith,LessonPlanet/wraith,BBC-News/wraith,marciovicente/wraith,jzinkdi/wraith,LessonPlanet/wraith,adouglas/wraith,LessonPlanet/wraith,marciovicente/wraith,jzinkdi/wraith,saulirajala/wraith,peruukki/wraith,jzinkdi/wraith,saulirajala/wraith,adouglas/wraith,peruukki/wraith,marciovicente/wraith,BBC-News/wraith