Ensure a stack exists before trying to destroy it.

Currently issuing a destroy for a stack that doesn't exist says the destroy was successful:

$ simple_deploy destroy -e env -n does-not-exist
2013-07-24 14:00:03 -0700 INFO : Cloud Formation stack destroy completed.
2013-07-24 14:00:04 -0700 INFO : Delete from SimpleDB successful.
2013-07-24 14:00:04 -0700 INFO : does-not-exist destroyed.