Enforced presence of a completedBlock in downloadWithURL:options:progress:completed

This method bails out in case of a missing `completedBlock`. While this makes sense (downloading the image and not doing anything with it is pointless), a client passing a `nil` block is not informed about the mistake.

`NSParameterAssert` validates the input and informs the client about bad usage of the API.
I understand that case. But we should think about most of the developers that are used the possibility to pass nil as a completionBlock, even for Apple APIs. I don't think we have a clear way to explain to them that they should be using the prefetcher in this case.