Fixed screen reader issues.

Tickets: [TNL-568] (https://openedx.atlassian.net/browse/TNL-568), [TNL-588] (https://openedx.atlassian.net/browse/TNL-588), [TNL-595] (https://openedx.atlassian.net/browse/TNL-595)

Removed unnecessary ```aria-hidden="true"``` attribute and added ```aria-live="polite"``` for problems.