gpu: Report renderer errors with exceptions

Instead of using a two step initialization to report errors, initialize
the GPU renderer and rasterizer on the constructor and report errors
through std::runtime_error.

src/video_core/gpu.h

@@ -366,6 +366,7 @@ protected:
     std::unique_ptr<Tegra::DmaPusher> dma_pusher;
     std::unique_ptr<Tegra::CDmaPusher> cdma_pusher;
     std::unique_ptr<VideoCore::RendererBase> renderer;
+    VideoCore::RasterizerInterface* rasterizer = nullptr;
     const bool use_nvdec;
 
 private: