fix alerts.js error with non-existent roles

test plan:
* create a custom account role
* enable alerts for the account
* create an alert referencing the custom role
* delete the custom role
* visit the accounts settings page using optimized js
e.g. "accounts/:id/settings?optimized_js=1"
* settings should still load

fixes #CNVS-12655

Change-Id: I0b9d86e8d331189836e5def4407fb155003cf8d7
Reviewed-on: https://gerrit.instructure.com/33961
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>