Determine what should happen if a stream handler throws an exception

[FrancoisChabot]

My immediate intuition is that it should go something like this:

  Stream_promise<int> prom;
  auto fut = prom.get_future();

  int total = 0 ;
  auto done = fut.for_each([&](int v) {
    total += v;
    throw std::runtime_error("bam!");
  });

  prom.push(1);
  prom.push(1);

  EXPECT_EQ(1, total);
  EXPECT_THROW(done.get(), std::runtime_error);