Remove special "KeyError" and return undefined from stores for missing key

[manzt]

Adapted from: gzuidhof/zarr.js#69

A KeyError is a built in python error that's thrown when a key is missing from a MutableMapping. In JavaScript, the equivalent behavior is to just return undefined when a key is missing. Making this change would remove a lot of the try/catch blocks from the core implementation and instead we would just call:

// Store returns 'undefined' instead of throwing custom error if chunk is missing.
// Otherwise the store can throw any other error and it will propagate up.
const chunk = await this.store.get(ckey);
if (!chunk) {
  /* create the missing chunk */ 
} else {
  /* decode the chunk */
}

[manzt]

This means that custom store implementations don't need to have zarrita as a dependency, and instead can explicitly return undefined when a key is missing.

[manzt]

After experimenting with this for a bit, I'm not sure what the "right" approach is here. We would need to be careful in handling the store internally (and this could bite us), but custom store implementations would be much more simple:

1.) return bytes if key in store.
2.) return undefined if not in store.