In stream operations, if an exception occurs during evaluation, it is passed through the call stack back to the terminal operation. With the addition of Gatherer, user code is now part of the call stack, and such downstream exception emerge from the Gatherer.Downstream::push calls. It is possible that Gatherer implementations' exception handling block wrap these calls and accidentally swallow exceptions that should have been propagated. We should add a note to Downstream::push and other relevant APIs, to allow Gatherer writers better handle exceptions.