define .length for ActiveModel::Errors for rails 3

Change-Id: Iff18c336743dfe3492337bdf576ccec145b9e375
Reviewed-on: https://gerrit.instructure.com/29256
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>