Add ability to manually bypass failed tasks about dagobah

[rclough]

This would be a cool feature, here's 2 ideas I have for it:

1. A "start from step " option that starts any workflow from a given step
2. A "mark step as completed" option which might be cleaner. Ie after manual intervention, you mark the step as successful and the dependency chain continues as normal.

1 has additional use cases beyond failures, ie you have bad data from one point on so there's no explicit failure, but you dont want to run the whole job again. 2 is probably the simplest fix simply for manually correcting errors like in your use case.

[thieman]

I think we'd want to do a slight variant on 2. Instead of marking as completed, we should add a fourth task state representing a skipped job. Job states currently include waiting, running, and failed. These states are used to determine what actions are possible at any given time: https://github.com/thieman/dagobah/blob/master/dagobah/core/components.py#L49

I think the clarity of adding a fourth state would be worth the extra effort. It would be weird to get a job completed email that told you Task 7 was completed when in fact you skipped it.

You should be able to manually mark a task as skipped in the UI and then retry the job. Skipped tasks should be counted as "completed" for the purposes of graph traversal and job completion.