stage add: improve error for overlapping outputs

[dberenbaum]

If I already have model.pkl.dvc and try to add a stage with that as an output, I get:

$ dvc stage add -n train -d train.py -o model.pkl python train.py
ERROR: output 'model.pkl' is already specified in stages:
        - train
        - model.pkl.dvc

This is unhelpful for a couple reasons:

1. It's not already specified in train. That's what I'm trying to add.
2. It should provide instructions for how to remedy it, like Use 'dvc remove model.pkl.dvc' first to track 'model.pkl' as an output of stage 'train'.

This matters for a scenario like iterative/example-repos-dev#171. Someone starts using experiments without pipelines and adds the model to track in DVC. If they later want to add a pipeline, it's not clear how to do so.