locale: make errors in writing files not critical

locale: make errors in writing files not critical

Suggested at https://github.com/systemd/systemd/pull/25805#discussion_r1054871210

> we now write multiple files and we cannot possibly guarantee that
> either both updates worked or neither. unix fs apis simply have
> no concept that would allow us to implement that. hence I think
> we should update what we can, log about what we cannot, but return
> success as long as our in-memory state was updated.