Handle parse errors gracefully on timeline items

Prior to this commit, if even a single item fails to parse Invidious
will throw out an error. This means that even if everything else
on a page can be parsed and rendered without issues, the single
problematic item will cause the entire page to be unusable.

This commit gracefully handles parse errors by catching and then
replacing the problematic item with a new "timeline error" object
that represents the parse error. This will allow the rest of the page
to be rendered and an error card that will replace the location of the
problematic item.

src/invidious/helpers/serialized_yt_data.cr

@@ -291,6 +291,24 @@ struct SearchHashtag
   end
 end
 
+# A `ProblematicTimelineItem` is a `SearchItem` created by Invidious that
+# represents an item that caused an exception during parsing.
+#
+# This is not a parsed object from YouTube but rather an Invidious-only type
+# created to gracefully communicate parse errors without throwing away
+# the rest of the (hopefully) successfully parsed item on a page.
+struct ProblematicTimelineItem
+  property parse_exception : Exception
+
+  def initialize(@parse_exception); end
+
+  def to_json(locale : String?, json : JSON::Builder)
+    json.object do
+      json.field "type", "parse-error"
+    end
+  end
+end
+
 class Category
   include DB::Serializable
 
@@ -333,4 +351,4 @@ struct Continuation
   end
 end
 
-alias SearchItem = SearchVideo | SearchChannel | SearchPlaylist | SearchHashtag | Category
+alias SearchItem = SearchVideo | SearchChannel | SearchPlaylist | SearchHashtag | Category | ProblematicTimelineItem