When dataset not found

[arnodelorme]

Indicate so. Instead we get the error

Traceback (most recent call last):
  File "/Users/arno/GitHub/core_eeg/eegdash/examples/core/tutorial_age.py", line 56, in <module>
    ds_data = EEGDashDataset(
              ^^^^^^^^^^^^^^^
  File "/Users/arno/GitHub/core_eeg/eegdash/.venv/lib/python3.12/site-packages/eegdash/api.py", line 866, in __init__
    super().__init__(datasets)
  File "/Users/arno/GitHub/core_eeg/eegdash/.venv/lib/python3.12/site-packages/braindecode/datasets/base.py", line 443, in __init__
    super().__init__(list_of_ds)
  File "/Users/arno/GitHub/core_eeg/eegdash/.venv/lib/python3.12/site-packages/torch/utils/data/dataset.py", line 324, in __init__
    assert len(self.datasets) > 0, "datasets should not be an empty iterable"  # type: ignore[arg-type]
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: datasets should not be an empty iterable

[arnodelorme]

See #207