text: Title: improve error message when deepcopying dag
  Body: In some situations, the dag object may fail to deepcopy, throwing an uninformative error. we should catch it and show something like: "an error occurred when copying your DAG" 
  
  https://github.com/ploomber/ploomber/blob/afadae033e161090610692bfe4f95e941448ea05/src/ploomber/dag/dag.py#L680