chain() error handling

[rgarcia]

options that have been discussed offline:

1. Some way to get all the streams created out of a chain (like values()), DIY error handling from there.
2. Do it in toArray, toNull, etc. I think I've written this off in my mind since it doesn't come with a solution for chain()s you don't want to run or want to use as inputs elsewhere.
3. A version of chain() that lets you pass in a domain. As intermediate results are generated, they are added to the domain.