Handle missing chunks

[jhamman]

In Zarr's v2 spec, missing chunks are explicitly supported:

There is no need for all chunks to be present within an array store. If a chunk is not present then it is considered to be in an uninitialized state. An uninitialized chunk MUST be treated as if it was uniformly filled with the value of the “fill_value” field in the array metadata. If the “fill_value” field is null then the contents of the chunk are undefined.

With this in mind, this library should catch 404s and return chunk of data equal to fill_value.

cc @freeman-lab, @katamartin

[joshmoore]

Just a heads up that there are some ongoing discussions around fill_value defaults:

zarr-developers/zarr-specs#133