Improve wording in CLI exception when output path is ambiguous

Related to #929

DiscordChatExporter.Cli/Commands/Base/ExportCommandBase.cs

@@ -121,7 +121,8 @@ public abstract class ExportCommandBase : TokenCommandBase
         if (!isValidOutputPath)
         {
             throw new CommandException(
-                "Attempted to export multiple channels, but the output path is neither a directory nor a template."
+                "Attempted to export multiple channels, but the output path is neither a directory nor a template. " +
+                "If the provided output path is meant to be treated as a directory, make sure it ends with a slash."
             );
         }