Improve error message when deleting a Rest node

[apupier]

Please describe the feature that you want to propose

currently:

the rest entity doesn't have any steps and is not a container.

[lordrip]

Technically it's a collapsed container 😬 , we could probably extend the check to evaluate whether the container has children or not

[apupier]

Visually in the editor, there are no steps. So complicated to understand for a user.