StandardError is a fairly wide error class (it is the parent of many different errors).  I would recommend you exit with the specific error which Capistrano::CommandError

$ simple_deploy execute -e preprod_shared_us-west-1 -n lc-pod-2-dev-1-app-4 -c 'sldkfjsdlkfj'
2013-02-14 11:52:29 -0800 INFO : Setting command: 'sldkfjsdlkfj'.
/Users/bweaver/.rvm/gems/ruby-1.9.3-p327@simple_deploy/gems/capistrano-2.13.5/lib/capistrano/command.rb:176:in `process!': failed: "sh -c 'sldkfjsdlkfj'" on 50.18.245.9 (Capistrano::CommandError)