GraphListener gives you access to a FlowNode but not a StepContext which sometimes has richer information. It also provides no straightforward way to block the execution of a step before it begins (though you can abort the build soon afterward). For some purposes it would be nice to have something like

public interface StepListener extends ExtensionPoint {
  void newStep(Step step, StepContext context) throws Exception;
}

The natural place to call this would be right before Step.start, at which point all the abovementioned information is available, and any Exception can just be handled in onFailure just like a synchronous error from start would.