If a plugin's config is invalid, it will not crash Kibana until after Kibana connects to Elasticsearch. This doesn't necessarily cause any problems, but it is poor UX for sysadmins.

Currently, Kibana starts up in this order:

1. Plugin discovery runs
2. Kibana migrations are executed (which waits to establish a connection with Elasticsearch)
3. Plugin configs are validated & plugins are initialized

This causes the issue above where a config may be invalid, but Kibana will not crash with an error until after Elasticsearch has connected.

We can move plugin config validation to execute during initial plugin discovery here: https://github.com/elastic/kibana/blob/master/src/core/server/server.ts#L79