Downgrading kibana causes page to not load

[jbudz]

Found by @tsullivan

1. Clear .kibana
2. Start kibana 5.0.0
3. Change package.json.version to 5.0.0-alpha3
4. Restart server

kibana ids .kibana/config by the version in package.json. If we downgrade https://github.com/elastic/kibana/blob/master/src/plugins/elasticsearch/lib/upgrade_config.js#L35 returns early and we don't create a new .kibana/config/my-downgraded-kibana version. Then

kibana/src/ui/settings/index.js

Line 29 in 1cf2979

return client

gets a 404 and our promise is never resolved().