In this loop, we ought to close the websocket connection gracefully when
the StreamErrWrapper reaches EOF.

Previously, it's possible that that we drop the last few events or skip sending
the websocket closure. If `handler(handlerPipe)` returns and `cancel` is called,
before the loop here completes processing streaming events, the loop exits
prematurely without propagating the last few events.

Instead here, the loop continues until we hit `httpPipe` EOF (through
`decoder.Decode`), to ensure we process the events to completion.

refactor the api handling of `nomad exec`, and ensure that we process
all received events before handling websocket closing.

The exit code should be the last message received, and we ought to
ignore any websocket close error we receive afterwards.

Previously, we used two channels: one for websocket frames and another
for handling errors. This raised the possibility that we processed the
error before processing the frames, resulting into an "unexpected EOF"
error.