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/objnone.c

@@ -18,7 +18,7 @@ STATIC void none_print(void (*print)(void *env, const char *fmt, ...), void *env
 STATIC mp_obj_t none_unary_op(int op, mp_obj_t o_in) {
     switch (op) {
         case MP_UNARY_OP_BOOL: return mp_const_false;
-        default: return MP_OBJ_NULL; // op not supported for None
+        default: return MP_OBJ_NOT_SUPPORTED;
     }
 }