Allow task estimator to return errors

[jayshrivastava]

Ex. in task_estimation, I may want something like so

if let Some(my_custom_exec) = plan.as_any().downcast_ref::<MyCustomExec>() {
     ....
} else {
     internal_err!()
}

In case we want to error instead of falling back to single node mode.

[gabotechs]

This sounds like a fair ask, +1 to this