fix: refuse a platform scrape that lists no file

scraper_cli wrote a platform YAML with no system when the parse came back
empty, and exited 0. It now exits 1 before writing, unless the scraper
declares lists_files = False (RetroPie, whose files are inherited); RetroPie
in turn raises when its archive declares no package.

scripts/scraper/retropie_scraper.py

@@ -59,6 +59,8 @@ _PACKAGE_DIRS = _SCRIPT_SECTIONS
 class Scraper(BaseScraper):
     """Scraper for the RetroPie package list."""
 
+    lists_files = False
+
     def __init__(self, url: str | None = None) -> None:
         # The package list is read from master: the commit read is recorded.
         if url is None:
@@ -122,6 +124,8 @@ class Scraper(BaseScraper):
                 else:
                     found.add(match.group(1))
 
+        if not found:
+            raise ValueError(f"{self.url}: no rp_module_id under {SCRIPTMODULES}/")
         self._modules = sorted(found)
         return self._modules