Throw a video not implementedm error upon calling that method

For #774 :)

So right now this is straight on the DSL level (impl and specs)... This isn't exactly future proof as what is implemented and what is not is a matter of the backend. So _technically_ the error should be thrown upon instantiation of the appropriate backend object. However that makes it a bit harder to follow.. e.g. when you look at the code starting at DSL it's "Oh right the DSL object is created let's have a look there... oh cool it creates a backend object great... whooops that throws a NotImplementedError''

Open to changing that but looking for feedback :)

Tobi