A memo over latest(m) keeps m's last value when m starts throwing #4377

Found by the async app generator's E9 pair, which compares latest(x) read at a leaf with the same read inside a derivation (#4371). Reproduces on next 5f60713.

const [a, setA] = createSignal(0);
const m = createMemo(() => {
  if (a() === 1) throw new Error("boom");
  return a();
});
const d = createMemo(() => latest(m));

render(() => <Errored fallback={e => "[ERR]"}><span>{d()}</span></Errored>, el);        // derive
render(() => <Errored fallback={e => "[ERR]"}><span>{latest(m)}</span></Errored>, el2); // leaf

setA(1);
flush(); // then settle

• Leaf: [ERR]. This is correct, and it's what a fresh render shows.
• Derive: <span>0</span>, which never changes. d keeps m's last value instead of rethrowing m's error.

So m is synchronous, but reading it through latest() inside a memo swallows its error. In the generator this is the most common E9 signature: 7 of 1,000 cases.