#2579 update: an engine binary without its BUILD.json is never updated, and nothing says why (follow-up to #2054)

说明

Follow-up to #2054 (the maintainer asked for it as its own issue). Related: #2159 (an interrupted update that
leaves `BUILD.json.prev`, PR #2161), which covers one way this state happens.

**What happens.** With `engine/BUILD.json` gone (a user "cleaning up", or #2159's interrupted update) and the engine
binary still there, `./update.sh` / `UPDATE.bat` neither downloads nor compiles an engine and says nothing about
why. In #2054 it only printed the binary's old version and finished. There is no state in which the update offers to
rebuild.

**Where.** `setup.py:3612` (`update_installed_engine`, `296b59d2`): `if not info.exists() or not (eng /
EXE).exists(): return`.

**Possible change.** Treat a binary without its stamp as of unknown origin and offer to replace it (a download of
the ready-made engine, or a compile with `--build`). Or at least print one line saying that `BUILD.json` is missing,
so the engine is not updated, and how to get a new one.