terminateInstances doesn't handle InvalidInstanceID.NotFound

[jonasfj]

See:
https://sentry.prod.mozaws.net/operations/ec2-manager/issues/1353073/

When people sheriffs kill bad instances, and they happen to be gone, they get a 500 error.
If the instanceId doesn't exist, we should probably just return 200 ok... as deleting an instance that doesn't exist seems like a successful noop.

I suspect this can be a race condition where people click multiple times, or the instance is racing to kill itself while people are killing it... Hence, by just saying a missing instance is a successful deletion, we get an idempotent operation.