Show friendly error when tries to record already existed dataset

[helper2424]

What this does

This PR makes the process of dataset recording is more friendly. If you try to run scripts for recording dataset two times sequentially (it could happend because of some env error, or robot error or whatever) the script will show you the following error:

    obj.meta = LeRobotDatasetMetadata.create(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/helper2424/Documents/lerobot/lerobot/common/datasets/lerobot_dataset.py", line 291, in create
    obj.root.mkdir(parents=True, exist_ok=False)
  File "/opt/homebrew/Cellar/python@3.12/3.12.7_1/Frameworks/Python.framework/Versions/3.12/lib/python3.12/pathlib.py", line 1311, in mkdir
    os.mkdir(self, mode)
FileExistsError: [Errno 17] File exists: '/Users/helper2424/.cache/huggingface/lerobot/helper2424/hil-serl-pusht-reward-classifier-test'

It's difficult to realize what it means. The answer is easy - we try to create dataset two times. If it exists the code will crashs.

To avoid the confusion I have added error handling and user friendly error text which asks user to launch the script one more time with --resume flag.

Examples:

Before	After

How it was tested

I have launched two times the following command

python lerobot/scripts/control_robot.py record \    --robot-path lerobot/configs/robot/koch_helper2424.yaml \
    --num-episodes 2 \
    --warmup-time-s 5 \
    --episode-time-s 10 \
    --fps 30 \
    --policy-overrides "device=mps" \
    --repo-id ${HF_USER}/hil12-serl-pusht-reward-classifier-test \
    --single-task "Push small T object to the correct position"

Unit tests:

pytest tests/lerobot/common/datasets/test_lerobot_dataset.py