[Rollups] Improve error when "Refresh jobs" request fails due to server being unavailable

[bhavyarm]

Kibana version: 6.5.1 BC2

Elasticsearch version: 6.5.1 BC2

Browser version: chrome latest

Browser OS version: OS X

Original install method (e.g. download page, yum, from source, etc.): from staging

Describe the bug: If you are on roll up jobs screen and shut down Kibana - Kibana displays fatal error:

 error  [22:33:34.265] [error][implementation][internal] Error: stream.push() after EOF
    at readableAddChunk (_stream_readable.js:240:30)
    at StreamStream.Readable.push (_stream_readable.js:208:10)
    at StreamStream.Transform.push (_stream_transform.js:147:32)
    at ReadStream.onData (/Users/bhavyarajumandya/Desktop/BC2_6.5.1/kibana-6.5.1-darwin-x86_64/node_modules/stream-stream/lib/StreamStream.js:69:12)
    at emitOne (events.js:116:13)
    at ReadStream.emit (events.js:211:7)
    at addChunk (_stream_readable.js:263:12)
    at readableAddChunk (_stream_readable.js:250:11)
    at ReadStream.Readable.push (_stream_readable.js:208:10)
    at fs.read (fs.js:2058:12)
    at FSReqWrap.wrapper [as oncomplete] (fs.js:658:17)

Please note these log lines are seen while creating a rollup job. Happened while testing : #25405

Also I saw this when I shut down Kibana

[elasticmachine]

Pinging @elastic/kibana-management

[cjcenizal]

Thanks for spotting this @bhavyarm! This error is expected, but it can be improved. A more desirable UX would be to surface a warning toast stating that the server is unavailable instead of displaying the fatal error. I've updated this issue's title and labels to reflect this enhancement.