LASIM outputs errors every upload for communities/users of defederated instances

[CMahaff]

Describe the Bug
If your target instance has de-federated from an instance with communities you want to subscribe/block or users you want to block, you will get 400 errors every upload that it cannot find these communities/users, which logically makes sense, but might be confusing.

The existing profile might have the federation list (or we could fetch it), and then filter our communities that are de-federated (with a message to the user of course).

Information
LASIM Version: 0.2.1
LASIM Profile Version: 2
Lemmy Source Instance URL: N/A
Lemmy Source Instance Version: N/A
Lemmy Target Instance URL: N/A
Lemmy Target Instance Version: N/A

Output of LASIM
N/A