RFC: remove return_failed kwarg from FlowRunner.run()?

[jlowin]

The more that we run Flows in distributed contexts, and use helpers like raise_on_exception, the less useful I think the return_failed kwarg is for FlowRunner.run(). In particular, it requires some messy code because it can only be satisfied by gathering every single task state to the FlowRunner process, then filtering. If people are finding it useful, then I retract this, but IMO it is not as useful as it used to be.

[cicdw]

I only use raise_on_exception personally, so I'm OK with this

[jlowin]

I think an improvement would be to create a debug state_handler that added failed tasks to a global dictionary. 🤔