Possible outcomes:

* We are currently added to the staging app, in which case this prints out the processes for Heroku to `STDOUT`. Exit code: `0`.
* We are not currently added to the staging app, in which case this prints out ` !    You do not have access to the app APP_NAME.` to `STDERR`.
* I guess the connection could drop mid-request. I have no idea what happens in this case.

I don't want to see any output in either of the two cases that I know about. If I'm already added, then fine, do nothing. If I'm not (exit code 1), add me.