Backup method for alerting contact

Hi,

Currently, when using a contact for alerting, there is no backup option in case the email/json gateway that things get sent to is down.

It would be nice that you can configure another contact ID with a contact as a backup.

In our case for example, we use SMSeagle to send out alerts, so there is a json url posted. If that gives a timeout, because that sms eagle in a specific datacenter is down, no alerts will be sent. If we have an option for another contact ID there, specified as the backup, there is a chance, an alert will still be sent out.