Handle non-200 status codes with JSON payload in GitHub API responses

[zonia3000]

I encountered the following error on application startup:

thread 'main' panicked at src/main.rs:302:82:
called `Option::unwrap()` on a `None` value
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: core::option::unwrap_failed
   4: <pineflash::Flasher as eframe::epi::App>::update
   5: <eframe::native::glow_integration::GlowWinitApp as eframe::native::winit_integration::WinitApp>::run_ui_and_paint
   6: core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut
   7: winit::platform_impl::platform::x11::EventLoop<T>::run_on_demand
   8: eframe::run_native
   9: pineflash::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Debugging, I discovered that it was caused by the URL https://github.com/repos/Ralim/IronOS/releases replying as follow:

{
  "message": "API rate limit exceeded for XX.XX.XX.XX. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)",
  "documentation_url": "https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"
}

The code checks for invalid JSON but not for valid JSON with status code different than 200, as this case. I added a check for the status code too.