exp run: showing unhelpful error message for invalid pipeline

[dberenbaum]

Background in #8973.

Here is the setup from that issue:

# train.py
from random import random

from dvclive import Live

NUM_EPOCHS = 10


with Live(save_dvc_exp=True) as live:

    live.log_param("NUM_EPOCHS", NUM_EPOCHS)

    for epoch in range(NUM_EPOCHS):
        step = live.read_step()
        live.log_metric("metric", random())
        live.log_param("my_step", step)
        live.next_step()

# dvc.yaml
stages:
  train:
    cmd: python train.py
    deps:
      - train.py
    outs:
      - dvclive:
          checkpoint: true
    metrics:
      - dvclive/metrics.json
    plots:
      - dvclive/plots

When running dvc exp run, the error message looks like:

Expected

dvc repro for a similar setup gives an error like:

ERROR: The output paths:
'dvclive'('train')
'dvclive/metrics.json'('train')
overlap and are thus in the same tracked directory.
To keep reproducibility, outputs should be in separate tracked directories or tracked individually.

The same error from dvc repro should be shown in dvc exp run.

[dberenbaum]

@skshetry This is another issue to keep in mind re: #9370