refactor: Replace N/D with more verbose Unknown error

Review proposed "No error", but is misleading, as this fuction is only called where we know there was an error

src/cli/action_convert.cpp

@@ -618,5 +618,5 @@ static const vlChar* get_last_vtflib_error() {
 		return error + 7;
 	}
 
-	return "N/D";
+	return "Unknown error";
 }