Incomplete stack trace of errors thrown in async functions

```
import {contentView, Button} from 'tabris';

contentView.append(
Push
);

async function test() {
await new Promise(r => setTimeout(r, 100));
await test2();
}

async function test2() {
await new Promise(r => setTimeout(r, 100));
throw new Error('foo');
}
```
Prints:
```
Listener for Button event "select" rejected:
Error: foo
at ./test.jsx:14:9
```

Should print something like
```
Listener for Button event "select" rejected:
Error: foo
at ./test.jsx:9:3
at ./test.jsx:14:9
```

Tabris.js does keep snapshots of the stack trace at the moment a promise is created and sets it as "current" while the promise resolves, but when the error is formatted the promise has already resolved (rejected) and the snapshot is no longer "current"