No satisfying error handling when uploading a dataset with an empty layer

[cdfhalle]

Context

• Affected library: webknossos
  The error occurs when creating a Dataset that contains an empty layer using the WK python library and subsequently calling dataset.upload().
  Code to reproduce the error

import webknossos as wk
import numpy as np

def main():
    dataset = wk.Dataset("testoutput/emptyLayerDataset", voxel_size=(30, 30, 32.48))
    layer = dataset.add_layer(
        layer_name="color", category="color", dtype_per_channel="uint8", num_channels=3
    )
    mag1 = layer.add_mag("1")
    mag1.write(
        absolute_offset=(10, 20, 30),
        # assuming the layer has 3 channels:
        data=(np.random.rand(3, 512, 512, 32) * 255).astype(np.uint8),
    )
    layer = dataset.add_layer(
        layer_name="segmentation",
        category="segmentation",
        largest_segment_id="255",
        dtype_per_channel="uint8",
        num_channels=1,
    )
    remote_dataset = dataset.upload()
    url = remote_dataset.url
    print(f"Successfully uploaded {url}")

if __name__ == "__main__":
    main() 

Expected Behavior

A descriptive Error message like "The dataset contains an empty layer." should be thrown.

Current Behavior

The following error message appears:
FileNotFoundError: Cannot create read-only dataset, could not find data at https://data-humerus.webknossos.org/data/zarr/scalable_minds/emptyLayerDataset.