[Watcher] Support for "unknown" watch state

[alisonelizabeth]

If the last watch action status is unknown, then the overall watch status should also be marked as unknown.

This is a follow-up issue to address this PR comment.

Judging how action_status and watch_status hang together, it would be cool to be able to say that if the last action status was unknown, then the watch_status is unknown? Remaining in this state should be a red flag to the user + they can still use the UI, we should communicate it's a possible bug. This could be a separate unit of work to this fix though!

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)