Custom error obejct properties are not propagated to `originalError` from a rejected promise

[magicmark]

In graphql/graphql-js#251, we added originalError to be able to look at the original error object and read stuff from it.

Great! Inside of formatError, I want to be able to see a custom property on the original error object.

works (regular synchronous Error):

const resolvers = {
  foo: () => {
    try {
      throw new Error('yikes!');
    } catch (e) {
      e.foo = "bar";
      throw e;
    }
  }
};

formatError: e => {
  console.log("originalError", e.originalError.foo); // logs 'bar'
  return e;
}

does not work (async Promise.reject):

const resolvers = {
  foo: async () => {
    try {
      await Promise.reject("yikes!");
    } catch (e) {
      e.foo = "bar";
      throw e;
    }
  }
};

formatError: e => {
  console.log("originalError", e.originalError.foo); // logs 'undefined'
  return e;
}

Repro: https://codesandbox.io/embed/nodejs-graphql-server-xo0vh

Not sure if this is intended, or if this is to do with there being some difference between an error object and a rejected promise, but we seem to treat rejected promises and synchronous errors the same elsewhere 🤔

Thoughts?

Thanks!!

[magicmark]

Actually guess i'll remake this issue at https://github.com/graphql/graphql-js