py: Add MP_OBJ_STOP_ITERATION and make good use of it.

Also make consistent use of MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL.
This helps a lot in debugging and understanding of function API.

py/objbool.c

@@ -48,8 +48,7 @@ STATIC mp_obj_t bool_binary_op(int op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
     if (MP_BINARY_OP_OR <= op && op <= MP_BINARY_OP_NOT_EQUAL) {
         return mp_binary_op(op, MP_OBJ_NEW_SMALL_INT((machine_int_t)mp_obj_is_true(lhs_in)), rhs_in);
     }
-    // operation not supported
-    return MP_OBJ_NULL;
+    return MP_OBJ_NOT_SUPPORTED;
 }
 
 const mp_obj_type_t mp_type_bool = {