exp run -n: check for conflicts before running experiment #6674
Description
dvc exp run -n will throw an error if a naming conflict arises, but it won't check until after the experiment has completed.
I guess someone could re-run the experiment later with a new name and the run-cache should prevent the whole experiment from needing to be executed, but this doesn't seem intuitive, and it would be better to know immediately that the experiment name conflicts with an existing experiment.
Steps to reproduce:
$ dvc init
$ echo "p: 0" > params.yaml
$ dvc stage add -p p -n test "echo test"
$ dvc exp run -n test
Running stage 'test':
> echo test
test
Generating lock file 'dvc.lock'
Updating lock file 'dvc.lock'
To track the changes with git, run:
git add dvc.yaml dvc.lock params.yaml
Reproduced experiment(s): test
Experiment results have been applied to your workspace.
To promote an experiment to a Git branch run:
dvc exp branch <exp> <branch>
$ dvc exp run -n test -S p=1
Running stage 'test':
> echo test
test
Updating lock file 'dvc.lock'
To track the changes with git, run:
git add params.yaml dvc.yaml dvc.lock
ERROR: Reproduced experiment conflicts with existing experiment 'refs/exps/d4/5e189b914d70882bd67b041110f3f27a30f941/test'. To overwrite the existing experiment run:
dvc exp run -f ...
To run this experiment with a different name run:
dvc exp run -n <new_name> ...