fallback on activerecord.errors translation scope for rails 3

Change-Id: I01c2a9a5e55eaa35ae377dc27a9d80fb9d42ad8a
Reviewed-on: https://gerrit.instructure.com/31306
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>