scheduler might keep reporting error when task cancelled before running

[D3Hunter]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

• start a import task
• dispatcher try move to next step, and save all subtasks into db(add subtask and update task is not in one txn, to handle the case that txn is large and we can add subtasks in batch)
• before dispatcher update task status and step, the task is cancelled
• dispatcher will generate revert subtasks for current step, which is stepInit
• import into didn't have subtask executor for init step, add index too.

[onError] [task-id=1] [error="unknown step -1 for import task 1"]

2. What did you expect to see? (Required)

cancelled

3. What did you see instead (Required)

scheduler keeps reporting error

4. What is your TiDB version? (Required)

master