Always check whether stream-level SENDMEs are expected.
(It's a protocol violation to get a SENDME when our send window is already full.)
This patch makes SendWindow::put return a Result, so that it's easier to do the right thing with it.
(NOTE: This change currently makes the tests fail because of #260 (closed). The current Arti code doesn't actually decrement stream windows, and as such doesn't notice invalid stream-level SENDME cells. Let's not merge this until #260 (closed) is fixed.)
Closes #261 (closed).