Impossible to remove empty cluster contexts

[ceresek]

It is possible to have contexts that are marked as RUNNING but have no running tasks in them (probably this can happen when an exception is thrown in the generator task). These contexts are displayed in the UI and cannot be removed - the Remove button is not displayed (because the contexts are still running), and the Kill button does nothing (because the contexts have no tasks to kill and hence the TaskContextCheckerAction, which would probably mark the context as finished, never runs).