Functional dashboards even if exporter is non-functional

Currently we rely on both Prometheus, the Couchbase Exporter, and the Cluster Monitor JSON API all working to render the cluster overview dashboard - in particular to find the list of clusters to populate the $cluster variable. If the exporter is non-functional, the entire dashboard breaks.

Investigate if there's a way to remove this dependency - perhaps loading check results entirely from the JSON API, or have a "your exporter is misconfigured" tile.