fix: exit 1 when a hash lookup finds nothing

The documented example looked up a test fixture hash, printed NOT FOUND
and exited 0. The docs now use a hash the database holds and say which
line forms a hash file accepts.

scripts/generate_pack.py

@@ -3033,9 +3033,11 @@ def _main(held: contextlib.ExitStack) -> None:
             sys.exit(1)
         db = load_database(args.db)
         if not args.platform and not args.emulator:
-            lookup_hashes(
+            # A lookup that found nothing is an answer a script must see.
+            if lookup_hashes(
                 hashes, db, args.bios_dir, args.emulators_dir, args.platforms_dir
-            )
+            ):
+                sys.exit(1)
             return
         zip_contents = build_zip_contents_index(db)
         with _pack_output_lock(args.output_dir):