Workflow should throw meaningful error on Node destruction

[CherryPerry]

Throws proper.

Closes #350, see comments.

[CherryPerry]

@zsoltk I have split executeWorkflow into executeWorkflow and maybeExecuteWorkflow. It is also possible to do executeWorkflow(strict: Boolean, action: () -> Unit) but in this case we need to change public API and client code to make it Maybe instead if Single (loosing backward compatibility and migration is required).