more informative error message
more informative error message

dhgarrette/cnn,Peilun-Li/dynet,Abasyoni/dynet,dhgarrette/cnn,mitchellstern/dynet,Peilun-Li/dynet,mitchellstern/dynet,cherryc/dynet,kamigaito/cnn,Abasyoni/dynet,cherryc/dynet,kamigaito/cnn,Abasyoni/dynet,Peilun-Li/dynet,dhgarrette/cnn,cherryc/dynet