all: Use MP_ERROR_TEXT for all error messages.

py/objset.c

@@ -362,7 +362,7 @@ STATIC mp_obj_t set_pop(mp_obj_t self_in) {
     mp_obj_set_t *self = MP_OBJ_TO_PTR(self_in);
     mp_obj_t obj = mp_set_remove_first(&self->set);
     if (obj == MP_OBJ_NULL) {
-        mp_raise_msg(&mp_type_KeyError, "pop from an empty set");
+        mp_raise_msg(&mp_type_KeyError, MP_ERROR_TEXT("pop from an empty set"));
     }
     return obj;
 }