Make sure napari can open compact sharded data

[melissalinkert]

See error noted in #290 (comment) with the output of bioformats2raw test\&pixelType\=uint8\&sizeX\=10000\&sizeY\=10000.fake test-v3-shard2048-compact.zarr -p --ngff-version 0.5 --shard-width 2048 --shard-height 2048 --compact.

The napari-zarr.py code referenced at the top of the stack trace would be useful to investigate further.