Follow-up from "Draft: arti-client: sketch out an initial "launch an onion service" API."

The following discussion from !1620 (merged) should be addressed:

• @gabi-250 started a discussion:

      // TODO HSS: This is actually a PublisherError, but that type isn't exposed,
      // and it contains a whole ecosystem of other crate-internal errors.
      // Either we should change Publisher::launch() to return a StartupError,
      // or we should figure out how much of PublisherError to expose.