log unmarshalling errors in encrypted_cookie_store

Change-Id: Idf8de2200b3f399f7e0dec7b5262678bd63248be
Reviewed-on: https://gerrit.instructure.com/39298
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>