fix potential xss vector in screenreader gradebook, fixes CNVS-17660

i haven't been able to exploit this yet, but that doesn't mean it's not
possible ;) ... make sure we sanitize the error messages we're putting
in here

Change-Id: I0a66696101834c6872d3b9c6e48a09c6525f6364
Reviewed-on: https://gerrit.instructure.com/46289
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>