Provide meaningful error message when URL is invalid

[eculver]

If users configure the URL incorrectly, messages like the following will emerge:

Unable to publish dashboard
{ error: 'RemoteTransportException[[myhost.dc][inet[/173.3.2.1:9300]][indices:data/write/index]]; nested: MapperParsingException[failed to parse [dashboard]]; nested: ElasticsearchIllegalArgumentException[unknown property [id]]; ',
  status: 400 }
Got statusCode400

The app should translate this into something meaningful to the user so that it's obvious what the problem is and how to fix.