Add an exception message

Author: flagbug

YoutubeExtractor/YoutubeExtractor/Decipherer.cs

@@ -142,7 +142,7 @@ private static string ApplyOperation(string cipher, string op)
                     }
 
                 default:
-                    throw new NotImplementedException();
+                    throw new NotImplementedException("Couldn't find cipher operation.");
             }
         }