datastore: Indication whether delete command was a no op or not

**Is your feature request related to a problem? Please describe.**
When operating a delete command, it would be useful if there was a way to know if the command had an effect or if it was a no op (ie, the key being deleted didn't exist).

**Describe the solution you'd like**
Ideally `ErrKeyNotExists` or something like it would be returned, but that would be a breaking change. In the node client it appears there is a callback that gives you the ability to make a check by seeing the number of index updates that occured, ie:
```
function deleteOrThrow(keyOrKeys, cb) {
datastore.delete(keyOrKeys, function (err, res) {
if (!err && res.indexUpdates === 0) {
err = new Error('Nothing was deleted!');
}
cb(err, res);
});
}
```

**Describe alternatives you've considered**
I would need to do a get to verify the key exists before doing the delete. I would prefer to not do 2 roundtrips to datastore for a single delete.