Unexpected error message when data source is unreachable

I successfully created a JSON index file for a NetCDF4 dataset using SingleHdf5ToZarr. I wanted to see what would happen if I intentionally changed the templates.u URI reference in the JSON file to a non-existent URI. This would simulate a situation where the dataset is moved after the index is created.

The index file is then read using this code:

import fsspec
import xarray as xr
import matplotlib.pyplot as plt

mapper = fsspec.get_mapper('reference://', fo="output2.json")

ds = xr.open_zarr(mapper, decode_times=False)

subset = ds['hus'].isel(plev=18).isel(time=1)
print(subset)

subset.plot()
plt.show()

I was expecting some kind of "file not found" type of error on line 7 (on open_zarr) but instead I got a ValueError when calling subset.plot() on line 12. The start of the error message is: "ValueError: The input coordinate is not sorted in increasing order along axis 0.".

Line 10 prints out the subset object and shows the following:

Coordinates:
  * lat      (lat) float64 nan nan nan nan nan nan ... nan nan nan nan nan nan
  * lon      (lon) float64 nan nan nan nan nan nan ... nan nan nan nan nan nan
    plev     float64 nan
    time     float64 45.0

So it seems that instead of failing on opening the (unreachable) data file, it's trying to create the subset object but assigning nans to the coordinate values. (Curiously, it's not assigning nan to the time coordinate.)

Start by running the provided Python snippet with the modified output2.json and trace the reference:// mapper through xr.open_zarr and the later subset access. Compare where the unreachable templates.u URI is read with when the NaN coordinates appear. Done should establish a clear, file-not-found-style failure instead of silently producing invalid coordinates, with regression coverage for this scenario.