Fix errors when deleting a forked project
The problem would occur when the `ForkedProjectLink` was deleted, but the `ForkNetworkMember` was not. The delete would be rolled back and retried. But the error would not be saved because `Project#forked?` would still be true, because the `ForkNetworkMember` exists. But the `Project#forked_project_link` would be `nil`. So the validation for the visibility level would fail.