Accessibility: Errors on the login screen are now read to screen reader users

Fixes CNVS-12223

test plan:
- Using a screen reader, visit the login page and trigger an error.
- Confirm that the error ("invalid password" or similar) is read automatically.

Change-Id: I0e9f19943a6bcdd78a73949cba58b88531e6eaf0
Reviewed-on: https://gerrit.instructure.com/35617
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>
QA-Review: Ryan Florence <ryanf@instructure.com>