rails3: the i18n scope for AR error messages changed

Change-Id: Ia739646db69ceec430a1968315d8191221e49d7e
Reviewed-on: https://gerrit.instructure.com/30792
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>