UnhandledPromiseRejection after sending response sometimes results in 502 error

[PeterBurner]

Description

I think there exists a timing problem between sending the HTTP response passed to express and promise rejections happening afterwards.
See this pseudo code:

app.get('/', async (req, res) => {
    const promise1 = Promise.reject({promise: 1});
    const promise2 = Promise.reject({promise: 2});

    let result;
    try {
      result = await promise1;
    } catch (e) {
      console.error(e);
      return res.status(400).end();
    }

    try {
      return res
        .status(200)
        .json(await promise2)
        .end();
    } catch (e) {
      console.error(e);
      return res.status(400).end();
    }
  }
);

I have an AWS Lambda behind an AWS API Gateway with a similar structure (my promises fetch HTTP resources). When the request handler returns before the second promise is handled there will be an UnhandledPromiseRejection. Apparently UnhandledPromiseRejections terminate the AWS Lambda execution (see https://forums.aws.amazon.com/message.jspa?messageID=906365).
On client side I sometimes get a 400 (--> my response) and sometimes a 502 (--> AWS response) status code.
With the workaround mentioned in the link above I always get the desired 400 response.

process.removeAllListeners('unhandledRejection');
process.on('unhandledRejection', (err) => console.error(err));

I don't know if the workaround is a good way to handle this situation but I think one solution or another should be part of this library. Of cause UnhandledPromiseRejections should only be caught after a HTTP response was passed to express.