handle basic request errors or update README with a demonstration

an error is seen in the browser tools using the below call `TypeError: Error in body stream`
```javascript
await new fetchEventSource( `${endpointUrl}/path-does-not-exist`, {
onerror ( err ) {
console.log( 'error', err );
},
onclose ( ev ) {
console.log( 'close', ev );
}
});
```
How does one get error details, such as a response code 404, 401 or 400? Thank you :)