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.
The reason `completedBlock` is required is exactly to prevent such kind of API misuse.

If you want to prefetch the images so that they will be a cache hit in the future, use `SDWebImagePrefetcher` instead.