Correctly sort elements

WebsiteGenerator/Content/Element.swift

@@ -258,7 +258,7 @@ extension Element {
         }
     }
 
-    private var sortedItems: [Element] {
+    var sortedItems: [Element] {
         if useManualSorting {
             return shownItems.sorted { $0.sortIndex! < $1.sortIndex! }
         }
@@ -308,7 +308,8 @@ extension Element {
      */
     func thumbnailFilePath(for language: String) -> String {
         guard let thumbnailFile = Element.findThumbnail(for: language, in: inputFolder) else {
-            fatalError()
+            log.add(error: "Missing thumbnail", source: path)
+            return Element.defaultThumbnailName
         }
         return pathRelativeToRootForContainedInputFile(thumbnailFile)
     }