add invalid subtask in case of invalid task factory config

Before the change, missing preload-task in task factory makes the parent
task invalid, which is not consistent with how invalid config is handled
in case of subtasks, subtask files and subtask externals. Handle
invalid task factory config by adding invalid subtask to make the logic
consistent with the rest.

Change-Id: I88288e6709c2667c689a2bd0b9c1531e18ff68a5
3 files changed