Better alarm on dead letter queue depth

[rclark]

There is an alarm when the dead letter queue's number of visible messages is greater than 0. This means you should get an email notification when the number increases from 0 --> 1.

It also means you should not get an alarm when that number increases from 1 --> 2, or any time after the initial failure. Perhaps we should write code in watchbot to manually trip the alarm and send an email every time a message goes to dead letter?

cc @ianshward @mapsam