The display is never cleared on re-evaluation, if a cell doesn’t display anything

If you have this cell:

```js
throw new Error();
```

And you edit it and write:

```js
if (false) display("success");
```

Then re-evaluating the cell continues to show the old error forever. The same is true if it displayed something:

```js
display("stale");
```

We should check when the cell resolves, and clear the display if it hasn’t been cleared already.