Abort a rake task when missing db/structure.sql like `db:schema:load` task.

When investigating #10381, I found this issue.

If you execute `db:structure:load` and you don't have `structure.sql`, you'll see an error message. But a rake task will not stop.

BTW, I've this for master (rails 4.1 stable ?). But I guess that this is bug fix for rails 4.0.x.