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.
If `SDWebImagePrefetcher` and this method do exactly the same thing but this method requires the block, then maybe we should only keep only one of the methods.