Improve error handling when meeting an unknown attribute.
Improve error handling when meeting an unknown attribute.

artas360/pythran,pombredanne/pythran,pbrunet/pythran,hainm/pythran,artas360/pythran,pbrunet/pythran,pbrunet/pythran,serge-sans-paille/pythran,pombredanne/pythran,pombredanne/pythran,artas360/pythran,hainm/pythran,serge-sans-paille/pythran,hainm/pythran