fix(youtube): a related page is empty only on YouTube's own word

The watch frame alone read as "nothing related" — so a related column YouTube moved, or a list
drawn by renderers this does not read, answered an empty list instead of an error. Now only the
"isn't available" promo YouTube draws for a video it no longer has is an empty page; anything else
it cannot read is a ScrapeError.

The flattening of a video renderer and the read of the continuation token are one each, in
innertube, for the search and the related scrape.

backend/tests/test_related_scrape.py

@@ -191,6 +191,26 @@ def test_a_video_youtube_no_longer_has_is_an_empty_page(net):
     assert related_scrape.related("aaaaaaaaaaa") == {"items": [], "next_page_token": None}
 
 
+def test_a_watch_frame_with_no_related_column_and_no_word_why_is_an_error(net):
+    # The frame alone, its related column moved somewhere this does not look: a lost shape, not
+    # "nothing related".
+    page = copy.deepcopy(FIRST)
+    watch = page["contents"]["twoColumnWatchNextResults"]
+    watch["sideColumn"] = watch.pop("secondaryResults")
+    _answer(net, page)
+    with pytest.raises(related_scrape.ScrapeError):
+        related_scrape.related("SdTZAMDKrNY")
+
+
+def test_a_related_list_drawn_by_renderers_this_does_not_read_is_an_error(net):
+    page = copy.deepcopy(FIRST)
+    results = _results(page)
+    results[:] = [{"videoTileViewModel": entry} for entry in results]
+    _answer(net, page)
+    with pytest.raises(related_scrape.ScrapeError):
+        related_scrape.related("SdTZAMDKrNY")
+
+
 @pytest.mark.parametrize(
     "answer",
     [