ganeti-netbox-sync: fix dry-run behaviour

• The script was failing with an unclear message in dry-run mode.
• Make the script fail explicitely with a clearer message as it can't fully run in dry-run mode due to the fact that it will skip the creation of cascading dependent resources.

Bug: T314794
Change-Id: I00c4f9b2f26fd6bf60871c1120abc99e5919e2d9