[Rollups] More robust crud app API error handling

From review comments:
https://github.com/elastic/kibana/pull/21117#discussion_r227631870
https://github.com/elastic/kibana/pull/21117#discussion_r227632306
https://github.com/elastic/kibana/pull/21117#discussion_r227632491
https://github.com/elastic/kibana/pull/21117#discussion_r227633784

cc @cjcenizal 