src: prepare for V8 Swallowed Rejection Hook

This is done in preparation for landing

  https://chromium-review.googlesource.com/c/v8/v8/+/1126099

on the V8 side, which extends the existing PromiseRejectEvent mechanism
with new hooks for reject/resolve after a Promise was previously
resolved already.

Refs: nodejs/promises-debugging#8
Design: https://goo.gl/2stLUY

PR-URL: #21838
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>

src/bootstrapper.cc

@@ -69,7 +69,7 @@ void PromiseRejectCallback(PromiseRejectMessage message) {
     callback = env->promise_reject_handled_function();
     value = Undefined(isolate);
   } else {
-    UNREACHABLE();
+    return;
   }
 
   Local<Value> args[] = { promise, value };