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.
Please note also that this PR only made this design choice explicit: the previous implementation simply bailed out in case of a `nil` `completedBlock`, without warning about the API misuse.