fix(gpu_engine): handle gpu_engine not supported

[mmrrnn]

I'm using intel's integrated card so the app panics here:

    #[cfg(feature = "nvidia")]
    let mut gpu_engine = GpuEngine::new(CudaEngine::new());

    #[cfg(feature = "opencl3")]
    let mut gpu_engine = GpuEngine::new(OpenClEngine::new());

    gpu_engine.init().unwrap();

Since I haven't found any alternatives to address my situation, I have decided to simply return a print error and exit with code 1. This approach will resolve the issue of false positive detections, which currently lead to a continuous restart loop of the GPU miner in the Tari universe.