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.
Mahmood Ali