read error message screen readers when adding user

fixes CNVS-14078

If you're using VoiceOver or NVDA and trying to add a user to your
course but don't add a correct email, it won't read the error message.
This is now fixed.

Test Plan
Given I'm on the People page
And I'm using NVDA in FireFox
And I'm a Teacher
When I click the '+ People button
And I enter an email address that is not formatted correctly
And I press the "Next" button
Then I should see an error message explaining why the email didn't work
And the error message should have focus
And the error message should be read to the user automatically

Change-Id: Ic644bc9b5f93123bb9305a0d7c11e232d0c6620f
Reviewed-on: https://gerrit.instructure.com/38079
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Sterling Cobb <sterling@instructure.com>