fix: handle create/edit subscriber when target DG does not exist

[patriciareinoso]

Description

Network slices contain references to device groups. The webconsole allows you to create a NS with a reference to X Device Group before actually creating the Device Group. In this case, the DG will appear in the list of possible device groups to add a subscriber to.

If you try to add a subscriber to non-existent device group, the following errors will show up:

This PR handles the cases:

1. Create a subscriber in a non-existent DG
2. Edit a subscriber from a DG that no longer exists to some other DG
3. Edit a subscriber that is not associated to any DG
4. Edit a subscriber from some DG to a non-existing DG

If the target DG does not exist, the subscriber will not be created/edited and an error will be shown:

We can deal with the fact that the DG should not be shown in the first place here: #717

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.