show warning if elasticsearch is not reachable

## Current Behavior
no indicator is given in the UI that Elasticsearch is not reachable. That means by wrong configuration or because it is broken.

If Graylog is not (or would not) be able to ingest messages to Elasticsearch it should be visible with a clear meaningful error message.

## Context
This is connected to https://github.com/Graylog2/graylog2-server/issues/5812 but not the same as for the unexperienced user all looks right but the complete chain will not work because Graylog can't connect to elasticsearch with the current configuration - or it is not possible to ingest messages or create new indices.

Debugging this with users is painful and it would be much easier if Graylog already make a visible error about that.

## Your Environment

* Graylog Version: 3.0