Don't consider it an error if there is nothing to export (#1349)

DiscordChatExporter.Core/Exceptions/ChannelEmptyException.cs

@@ -0,0 +1,8 @@
+using System;
+
+namespace DiscordChatExporter.Core.Exceptions;
+
+// Thrown when there is circumstancially no message to export with given parameters,
+// though it should not be treated as a runtime error; simply warn instead
+public class ChannelEmptyException(string message)
+    : DiscordChatExporterException(message, false, null) { }