[Watcher UI] HTTP 400 error when watcher is disabled in ES x-pack

[elasticmachine]

Original comment by @ycombinator:

Kibana version: master

Elasticsearch version: master

Server OS version: macOS

Browser version: Chrome

Browser OS version: macOS

Original Kibana install method (e.g. download page, yum, from source, etc.): From source

Original X-Pack install method (e.g. by name, offline install, from source, etc.): From source

Description of the problem including expected versus actual behavior:

When Watcher is disabled in Elasticsearch, the Watcher UI continues to show. However, when attempting to create a watch, an HTTP 400 error is thrown.

Steps to reproduce:

1. Disable Watcher in X-Pack Elasticsearch by setting xpack.watcher.enabled: false
2. Restart ES and Kibana.
3. Go the Watcher UI in Kibana and try to create a watch

Errors in browser console (if relevant):

Nothing in the Dev Tools Console tab. See screenshot for HTTP 400 response in Dev Tools Network tab.

Provide logs and/or server output (if relevant):

Nothing unusual in the server logs.

[elasticmachine]

Original comment by @PhaedrusTheGreek:

I've hit this twice now - might be as simple as catching the 400 and suggesting that user check if watcher is enabled in elasticsearch.yml ?

[elasticmachine]

Pinging @elastic/es-ui

[cjcenizal]

Closing in favor of #43023