CDK-347. Implement DatasetTarget#handleExisting and WriteMode support.

Wouldn't it be better to check for data files in the view rather than assuming that the view exists if it is defined? I think checking for data would make this behave more like users expect, so they could use a view to assert that there shouldn't be any data for the portion they're writing. This is how I'd expect it to behave if I were writing versions, for example.