E(zoe).offer(...) should throw if offer handler throws

[dckc]

What is the Problem Being Solved?

You would think that the result of .offer() would be the offer result, but... nope! gotcha!
It's a seat, and if you forget to call E(seat).getOfferResult(), exceptions from the offer handler become unhandled rejections.

@LuqiPan got bit by this just as I did when learning the Zoe API:
Agoric/dapp-agoric-basics#8 (comment)

cc @erights @Chris-Hibbert @ivanlei

Description of the Design

• TBD

One idea is:

const { seat, result } = E(zoe).offer2(...)`

Replace offer2 with the bikeshed color of your choice.

But maybe an interim path to the target is a helper:

const { seat, result } = offer2(zoe, invitation, ...)

where offer2 is...

const offer2 = (zoe, ...args) => E(zoe).offer(...args)
  .then(seat => E(seat).getOfferResult().then(result => ({ seat, result })));

Security Considerations

Not sure, but my intuition says there's more than just unhandled rejections that can result from forgetting to wait for getOfferResult() to settle.

Scaling Considerations

n/a

Test Plan

• depends on design

Upgrade Considerations

adding the E(zoe).offer2(...) API would require a zoe upgrade.