Check for all downloaded pages during a chapter download

In case a chapter is marked as downloaded, but the download folder exists already, the chapter did not get downloaded again.
 This could cause issues in case the previous download was failed or has missing pages.
Instead of only checking if the folder exists, each page should be checked individually

This was previously done and was incorrectly changed with 1c9a139.

server/src/main/kotlin/suwayomi/tachidesk/manga/impl/download/fileProvider/impl/FolderProvider.kt

@@ -30,11 +30,6 @@ class FolderProvider(mangaId: Int, chapterId: Int) : ChaptersFilesProvider(manga
         val chapterDir = getChapterDownloadPath(mangaId, chapterId)
         val folder = File(chapterDir)
 
-        val alreadyDownloaded = folder.exists()
-        if (alreadyDownloaded) {
-            return true
-        }
-
         val downloadSucceeded = super.downloadImpl(download, scope, step)
         if (!downloadSucceeded) {
             return false