Additional debug info for GraphPipeline merge step failures

```
java.lang.IllegalStateException: Error during merging: Data instance already has key "default" and allowOverwrite is false

	at ai.konduit.serving.pipeline.api.data.Data.merge(Data.java:420)
	at ai.konduit.serving.pipeline.impl.pipeline.GraphPipelineExecutor.exec(GraphPipelineExecutor.java:132)
```

But which graph step? What inputs?
This is currently hard to debug.