AUI-1356 Make aui-form-validator accessible

Hey @marclundgren,

Here is an update for https://issues.liferay.com/browse/AUI-1356.

There's an issue with aria-describedby. Somehow the aria-describedby attribute is not working when you navigate to the input field with keyboard. The screen reader won't read out the information in the element indicated by aria-describedby. However, it will work if you click on it with mouse. I couldn't figure out what the problem is, and I'm wondering if it's just an implementation issue with screen reader.

Please let me know if there are any issues. Thanks!