fix: improve error messages when UPF/gnB lists are empty

[dariofaccin]

Description

This PR improves the error message handling during network slice creation. The error message is different whether the backend is not reached or the list of available UPFs/gnB is empty.

The following picture represents the new behaviour:

• UPF list is retrieved from the server and is empty
• gnB list has failed to be retrieved from the server

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.