[ML] Improve error message when transforms are not enabled on any node

[sophiec20]

Found in master "build_hash" : "5de0ed9432f4b41b697c9f3bc28953834cd3eae1", "build_date" : "2020-08-06T12:34:55.776311Z",

When node.roles: [ "transform" ] is not enabled on any node, then a transform cannot be run. The UI will let you step through the process of creating a transform, but the following error is displayed on start.

This node setting does not disable the API entirely, so GET _transform still works. But it does stop the transform from running on a node. If there are no nodes capable of starting the transform, then you will see this error.

Please can we improve the presentation of this error and make it more human read-able.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[walterra]

In #93956 we added better UI degradation when transform roles are missing. The "Create transform" button is no longer available in such a case and a warning callout explains the reasons.

Leaving this issue still open for now as we investigate a possible regression introduced by this PR (elastic/elasticsearch#70125) where the transform list would no longer load because the _transform/_stats endpoint is no longer usable without a transform role.

[walterra]

• This PR for Elasticsearch ([Transform] Make transform _stats work again, even if there are no transform nodes elasticsearch#72221) fixes the behavior of the _transform/_stats endpoint.
• This PR for Elasticsearch ([Transform] Enhance transform role checks  elasticsearch#70139) introduced a waiting state for transforms that cannot be assigned to a node. A corresponding Kibana issue to add the waiting state to the UI's stats bar has been added here: [ML] Transforms: Handle transforms in waiting state #98444