[Intermittent]Fail to terminate and restart task docker-build when the task is already active

[v-dantch]

OS: All Build Version: 20200122.2 Regression: Not a Regression

Repro Steps:

1. Create an Express App.
2. Build the Dockerfile --> Build the Dockerfile again immediately.
3. Pop up a warning "The task 'docker-build(appname)'is already active".
4. Select "Restart Task".

Expect: Succeed to restart task. Meanwhile there is a warning if fail to restart.

Actual: Only display "Failed to terminate and restart task docker-build" and there is no any log message.

[philliphoff]

It's possible our custom execution task isn't reacting to cancellation properly. We'll need to look into that.

[v-mengwe]

Verified on the latest build 20200818.1, this issue still reproduces for now. I will continue to track this issue.