[Uptime] Improve error handling when CCS is not enabled

[justinkambic]

We recently added the capability to configure the Uptime app's index pattern via kibana.yml. This works fine, but we also noticed that if the user is attempting to use cross-cluster search on a cluster that is not configured to use this feature, there is no helpful error message and the Kibana server log will flood with errors that are unrelated to the root cause.

In an attempt to abate the inevitable hard-to-debug forum posts and SDH tickets this could create in the future, we should create a better error handling experience around this specific problem. I mentioned this in the original implementation, and am creating this follow-up issue as an extension of the original comment.

[shahzad31]

this also happens if you will configure CCS via uptime settings page

[justinkambic]

@shahzad31 I have updated the issue's description to make it agnostic to the configuration method.