Can you import a Postgresql 9.5.10 db into a Postgresql 10.9 server?
The error message is harmless, it only means that you imported the dump as a non-superuser and the extensions already existed.

There are potential problems with using a non-superuser (ownership usually cannot be restored), but since you didn't get any such error messages, you are fine.

The only important consideration is that you need to use pg_dump from version 10 to dump the 9.5 database, not the 9.5 pg_dump. While v10 knows how to dump a 9.5 database so that it can be loaded into v10 properly, 9.5 lacks that knowledge.