fix active_model_errors for rails 3 compatibility

Change-Id: I2844bf39fed7694e8c34e1afde0a72299e86231e
Reviewed-on: https://gerrit.instructure.com/30693
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>