Better error handling when SAML realm is not available under current license

[ppf2]

Currently if the ES license does not allow for SAML realm, Kibana simply throws the error below when launching the base URL for Kibana:

{"statusCode":401,"error":"Unauthorized","message":"Unauthorized"}

The ES logs clearly shows the underlying issue:

[2020-03-16T23:28:12,135][INFO ][o.e.x.s.r.a.s.SamlBaseRestHandler] [node-1] The 'saml' realm is not available under the current license

It will be nice for Kibana to detect this condition and report an intuitive error message to the UI.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[legrego]

We could take advantage of the new Login Selector UI to show these options as disabled with a message indicating that they're not currently available. I know it's possible to disable the login selector, but this would be a good first step.

If the login selector is disabled, then we could show that generic "Login is disabled" screen, with a message to contact your administrator. We can report the real issue in the Kibana server logs at that point.