fix: task status issue with unpicklable artifacts

[saikonen]

if a flow contains unpicklable artifacts, the task status for such a step is recorded as successful even though pickling fails and leads to a non-zero exit for the task.

example:

from metaflow import FlowSpec, step, kubernetes

class DemoFlow(FlowSpec):

    @kubernetes
    @step
    def start(self):
        self.x = open("hello.txt", "w")
        self.next(self.end)

    @step
    def end(self):
        pass

if __name__ == "__main__":
    DemoFlow()

[talsperre]

Let's add self.flow._exception = MetaflowExceptionWrapper(ex) here as well. Maybe a print statement as well.

[saikonen]

what is the retained exception used for internally? I'm also not sure if adding a print will help in this case.

edit: re-raising the error correctly produces stderr that ends in task logs, so the user has an idea what went wrong with the flow