streamio: Fix partial writes and mishandling of read errors

While inspecting transaction-related failures in #3443 (comment 509073616), I was taking a look at whether gitaly-hooks may somehow fail to relay information properly. Turns out that in theory, it could be the root cause in some cases because we didn't correctly handle partial writes of the send buffer in streamio and didn't properly return errors when reading data. This MR fixes both cases.