Mark files that failed to download with an 'x' in the progress window. Also modified the legend to emphasize the fact that the file will again be tried to download during the next run.

src/main/kotlin/de/fabianonline/telegram_backup/DownloadManager.kt

@@ -364,8 +364,12 @@ class DownloadManager(internal var client: TelegramClient?, p: DownloadProgressI
 				prog!!.onMediaAlreadyPresent(m)
 			} else {
 				try {
-					m.download()
-					prog!!.onMediaDownloaded(m)
+					val result = m.download()
+					if (result) {
+						prog!!.onMediaDownloaded(m)
+					} else {
+						prog!!.onMediaSkipped()
+					}
 				} catch (e: TimeoutException) {
 					// do nothing - skip this file
 					prog!!.onMediaSkipped()