multipoolAutoswap needs to throw on seat.fail with an error argument

[katelynsills]

The contract code in multipoolAutoswap is incorrect:

https://github.com/Agoric/agoric-sdk/blob/9eebbe32309efc691570477f26f43cc49f49707c/packages/zoe/src/contracts/multipoolAutoswap/swap.js#L166:169

It should make an Error and include it as the argument to seat.fail and it should probably throw on that line as well. See

agoric-sdk/packages/zoe/src/contracts/simpleExchange.js

Line 128 in 9eebbe3

throw seat.fail(

for an example.

There may be a larger Zoe issue as well, but this isn't following the expectations that we have for Zoe contracts, which makes it worse than usual. We may need to compare Error messages to see why something has errored, but we certainly shouldn't be comparing offerResult strings to find an error - getOfferResult should throw. This problem in particular is a problem with how this contract was written.

Originally posted by @katelynsills in https://github.com/Agoric/dapp-token-economy/pull/112#discussion_r570624446