Warning prompt for saving search/dashboard/visualization with the same name is not announced in screenreader

[bhavyarm]

Kibana version: 6.3.0 BC6

Browser version: chrome latest

Browser OS version: OS X

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

Describe the bug: if you try to save a search/visualization/dashboard with the same name as one of the existing searches - Kibana warns the user. But this isn't announced to a non-sighted user.

Steps to reproduce:

1. Use voiceover
2. Go to discover and try to save a search with the same name as one of the existing searches
3. Kibana shows a warning prompt
4. But that action is not announced in screenreader

Expected behavior: Kibana needs to tell the user that he is saving the search with the same name.

Screenshots (if relevant):

Relevant WCAG Criteria: 4.1.3 Status Messages - Level AA