Overloaded per-item error codes (TyEPGNF / TyEPATNF_ for everything) and Scala internals leaking into messages

In get, the per-item error codes are overloaded to the point of being unhelpful, and a couple of messages leak Scala's case-class rendering.

Every per-item page failure comes back as TyEPGNF regardless of cause — including "unknown ref type" and "not an embedding url or discussion id," which aren't really "page not found." Pats similarly always come back as TyEPATNF_; the e2e test even comments that this code is used "whatever the error is."

And some messages expose internals: a missing pat comes back as e.g. Pat not found, ref: 'Username(no_such_user_xyz)' or SingleSignOnId(...) — that's the Scala case class toString, not the ref string the caller sent.

Who it bites: anyone who wants to branch on error code (can't — a malformed ref and a genuinely-absent page are indistinguishable) or show the failing ref back to a user (they get Username(...) noise). Cosmetic, but it makes API errors harder to handle programmatically and uglier to surface.