Report task information in addition to progress

It might be useful to include information about the number of tasks:

• active
• failed
• completed
• total

And not just the progress (completed/total). In a similar manner to what is done now, these could be summation of each category for all stages of the statements job. The API could then return something like:

{ ..., "progress": 0.8, "tasks": {"active": 2, "completed": 8, "total": 10, "failed": 0 } }

Users could then construct an indicator in the flavor of that in the spark UI.