[Dashboard] Correct the event time of failed tasks

[liuxsh9]

Why are these changes needed?

As shown in the figure below, for some failed tasks, their end time is incorrect.

Related issue number

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[rynewang]

Thanks, the change lgtm. I wonder if you can also help renaming the field to state_ns? Since this more accurately describes it

[rynewang]

@jjyao to merge.

Also TODO for us: make global change s/state_ts/state_ns/g

[jjyao]

Thanks for the fix.