Aborted render should mark render pass as converged

[nrusch]

Currently, performing a blocking Hydra render to completion requires a pattern similar to the following:

auto tasks = taskController->GetRenderingTasks();
do {
    engine.Execute(renderIndex, &tasks);
}
while (!taskController->IsConverged());

Right now, if the render performed by HdArnoldRenderPass aborts for any reason, the render pass is not marked as converged, and thus the above code will block indefinitely.

I think the most "correct" thing to do in the case of an Arnold render error is to mark the pass as converged and zero the render pass' output buffers.