Clarify error message.
Clarify error message.

Elasticsearch will throw a 404 if there is no indexes added yet, resulting in
this unclear message -- even for people that are not using any sort of proxy.

This message update should make it easier to quickly troubleshoot why the
the dialogue is popping up.

grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana