Fix the exception message

Author: flagbug

YoutubeExtractor/YoutubeExtractor/DownloadUrlResolver.cs

@@ -183,7 +183,7 @@ private static string GetStreamMap(JObject json)
 
             if (streamMapString == null || streamMapString.Contains("been+removed"))
             {
-                throw new VideoNotAvailableException("Video is removed");
+                throw new VideoNotAvailableException("Video is removed or has an age restriction.");
             }
 
             return streamMapString;