video_core: Cancel Scoped's exit call on GPU failure

When CreateRenderer fails, the GraphicsContext that was std::move'd into
it is destroyed before the Scoped that was created to manage its
currency. In that case, the GraphicsContext::Scoped will still call its
destructor at the ending of the function. And because the context is
destroyed, the Scoped will cause a crash as it attempts to call a
destroyed object's DoneCurrent function.

Since we know when the call would be invalid, call the Scoped's Cancel
method. This prevents it from calling a method on a destroyed object.