fix(ext/flash): Correctly handle errors for chunked responses

[kamilogorek]

The leading cause of the problem was that handleResponse has tryRespondChunked passed as an argument, which in turn is implemented as a call to core.ops.op_try_flash_respond_chuncked, that throws in the repro code.

handleResponse was not handled correctly, as it not returned any value, and had no catch attached to it.
It also effectively was never correctly handled inside two other blocks with resp.then and PromisePrototypeCatch(PromisePrototypeThen(resp, "...")) as well, as it just short-circuited the promise with an empty resolve, instead of relying on the last (async () => {}) block.

This change makes handleResponse return a correct value and attach onError handler to the "non-thenable" variant of response handling code.

continues are redundant, as there are no unhandled branches within the for loop at line 540.

---

Repro from #17291

addEventListener("unhandledrejection", (event) => {
  console.error("Unhandled rejection:", event.reason);
});

Deno.serve(
  (request) => {
    const stream = new ReadableStream({
      start(controller) {
          controller.enqueue("Hello world");
          controller.close()
      },
      cancel(reason) {
        console.log("Request cancelled:", reason);
      },
    });

    return new Response(stream, {
      status: 200,
      headers: {
        "Content-Type": "text/html; charset=utf-8",
      },
    });
  },
  {
    port: 9000,
    onError: (err) => {
      console.error("Server Error:", err);
      return new Response("Internal Server Error", { status: 500 });
    },
  }
);

$ deno run -A --unstable mod.ts
$ curl localhost:9000

Before:

Listening on http:https://127.0.0.1:9000/
Unhandled rejection: TypeError: Expected ArrayBufferView at position 2
    at tryRespondChunked (deno:ext/flash/01_http.js:643:35)
    at deno:ext/flash/01_http.js:393:17

After:

Listening on http:https://127.0.0.1:9000/
Server Error: TypeError: Expected ArrayBufferView at position 2
    at tryRespondChunked (deno:ext/flash/01_http.js:641:35)
    at deno:ext/flash/01_http.js:393:17

Fixes #17291

[bartlomieju]

Thanks @kamilogorek, any chance you could add a test that exercises this code path?

[kamilogorek]

Sure thing, done

[littledivy]

LGTM! Thank you