Raise NameError for symbol struct access.

mrbgems/mruby-struct/src/struct.c

@@ -399,7 +399,7 @@ struct_aref_sym(mrb_state *mrb, mrb_value obj, mrb_sym id)
       return ptr[i];
     }
   }
-  mrb_raisef(mrb, E_INDEX_ERROR, "'%S' is not a struct member", mrb_sym2str(mrb, id));
+  mrb_name_error(mrb, id, "no member '%S' in struct", mrb_sym2str(mrb, id));
   return mrb_nil_value();       /* not reached */
 }