Improve error reporting when the provider is not correctly configured.

[tobio]

From #354

We can't enforce these endpoints are configured within the provider schema since it should be possible to configure only the components you're using, however we should validate that the built clients have a valid base configuration (e.g at least one endpoint, and auth scheme).

[georgms]

As noted in #354 the kibana endpoints are required when configuring connectors or actions. If these are omitted, the error message is something like unsupported schema: "" so completely useless. Took me half a day to figure out that I was missing the kibana endpoints. It would be fantastic if you can improve the error reporting as discussed here. Thank you!