[ML] Validate Job modal gives no context as to the number or nature of errors

[aphelionz]

Steps to reproduce (assumes ChromeVox or similar)

1. Open Kibana in a new tab
2. Tab to Machine Learning
3. Tab to "+ Create New Job" button
4. Press enter
5. Choose the first saved search
6. Choose the "Single Metric" wizard
7. Fill out the form enough to enable the "Validate Job" button
8. Tab to the Validate Job Button
9. Press enter

Actual Result
10. A modal appears but it is not focused or read, making it extremely hard for screen reader users to learn what the modal is displaying.

Expected Result
10. Upon opening, there should be an indication that there is in fact a modal open, and at the very least summary info of what's inside

Category: #18866 Kibana 6.3 Meta Issues
Relevant WCAG Criteria: 3.2.2 Predictable on Input

[elasticmachine]

Pinging @elastic/ml-ui

[walterra]

There's an issue in EUI relevant to the Modal's accessibility here: elastic/eui#699 - Once this is resolved, the Modal's used in the ML plugin should also be fine.

[myasonik]

Closing this as there's nothing for ML team to do here