use RecordNotUnique, not UniqueConstraintViolation

since it's built into Rails 3

Change-Id: I55fba469a963538305f50b473bb05215ab5184d8
Reviewed-on: https://gerrit.instructure.com/40975
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>