Changeset not capturing constraint on deleting deployment

[lawik]

A customer hit this. You can't delete a deployment if it is used by devices. Fine. But the error needs to be graceful :)

lib/nerves_hub/deployments.ex in NervesHub.Deployments.delete_deployment/1 at line 116

Ecto.ConstraintError
constraint error when attempting to delete struct:

    * "devices_deployment_id_fkey" (foreign_key_constraint)

If you would like to stop this constraint violation from raising an
exception and instead add it as an error to your changeset, please
call `foreign_key_constraint/3` on your changeset with the constraint
`:name` as an option.

The changeset has not defined any constraint.