Improve error message when trying to stop nested service.

[sangee2004]

acorn version - v0.9.2-36-g3ff45929+3ff45929

Steps to reproduce the problem:

1. Deploy the my-todo app from getting started guide - https://docs.acorn.io/getting-started
2. Once the app is deployed successfully , try to stop the service db app.

User is presented with the following error message:

acorn stop my-todo.db
  ✗  ERROR:  stopping my-todo.db: App.api.acorn.io "my-todo.db" is invalid: metadata.name: Invalid value: "my-todo.db": To update a nested Acorn or a service, update the parent Acorn instead.

Expected Behavior:
This error message can be improved to state that nested Acorn or a service cannot be stopped when parent Acorn is still running.

[MuhtasimTanmoy]

Is this issue available or occupied?