Add propagated task_stop object

[Shians]

At the moment task_error objects are propagated for the purposes of recording errors in method calls. A class task_stop could be implemented with similar functionality but based on custom stop conditions.

For example, task_stop could be emitted based on a filtering step removing too many cells. Another use case would be if only the top 5/10 stepwise results based on some metric should be passed into the next method, a filtering function could replace all non-passing results with a task_stop that contains some information for the reason for stop.

The key functionality is that task_stop has the same functionality as task_error in terms of propagation, in that it is done automatically and individual methods do not need to know how to handle them.