Actions: Handle Action failure in the /job/action endpoint and UI

[philrenaud]

Currently, the nomad job action CLI command handles unsuccessful actions with an error. Try, for example, running an action to curl a 404ing url and compare the output of the operator API and the CLI:

nomad operator api \
-H "Connection: Upgrade" \
-H "Upgrade: websocket" \
-H "Sec-WebSocket-Version: 13" \
-H "Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==" \
-verbose \
'/v1/job/actions-demo/action?action=ipinfo&allocID=06273d00-579c-1415-be3a-444ae4bcc03c&task=task&tty=true'
> GET /v1/job/actions-demo/action HTTP/1.1
> Connection: Upgrade
> Upgrade: websocket
> Sec-Websocket-Version: 13
> Sec-Websocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
>
* Sending request and receiving response...
< HTTP/1.1 101 Switching Protocols
< Upgrade: websocket
< Connection: Upgrade
< Sec-Websocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
{"stdout":{"data":"Y3VybA=="}}R{"stdout":{"data":"OiAoNikgQ291bGQgbm90IHJlc29sdmUgaG9zdDogaXBpbmZvLmlvb28NCg=="}}{"stdout":{"close":true}}({"exited":true,"result":{"exit_code":6}}�[1m%                                              

nomad job action \
  -group=group \
  -task=task \
  -job=actions-demo \
  -t=true \
ipinfo
curl: (6) Could not resolve host: ipinfo.iooo

The socket provides an exit code in its exited frame, but that's not enough to show the wrapping connection as an error.

Provide a stderr message frame or otherwise return an error for the websocket connection.

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(Note! This may be front-end solvable: an action that fails a curl request/404s will send back an exit code of 6 like this: {"exited":true,"result":{}}, which we could respond to here)