fix(app): Raise error in case no processes are found in DeployStatus

[CuriousLearner]

We already check that the DeployStatus d is not nil, so checking if the processes within it are loaded and raise an error from FindProcess.

https://github.com/apppackio/apppack/blob/main/cmd/ps.go#L100-L101

[ipmb]

I'm not sure that the user should see an error in this scenario. If they should, that one won't make any sense to them.

The scenario is that they are trying to configure a review app before it has finished it's first deployment. The command causing the error was:

apppack -a iris:3561 ps resize web --cpu 1 --memory 2G

I can think of two possible responses here:

1. Exit with an error, "review app does not exist"
2. Mimic the behavior from fix(cmd/ps): resize raises warning for non-existent service #60

1 should happen if the review app stack does not exist. 2 should happen if the review app exists but the service does not.