[fix] UnhandledPromiseRejection crashes the koa server
## Node 16: UnhandledPromiseRejection crashes the koa server

**Node.js version:** v16.17.0

**Description:** When an UnhandledPromiseRejection occurs, the koa server crashes. I noticed that it seems to be on Node 16.
How I found this was while using a library in which they inadvertently caused an UnhandledPromiseRejection. Fortunately I caught this issue before experiencing in prod! 
I also sandbox tested with Express and it does the same. 

Using node 14 does not exhibit this behaviour which leads me to believe it's related to Node 16

## Code to reproduce

```const Koa = require('koa');
const app = new Koa();

app.use(ctx => {
    const prom = new Promise((resolve) => {
        throw 'Blah';
    });
});

app.listen(1234, () => {
    console.log('running on 1234')
});

