proper way to handle interface errors

interface.c

@@ -126,6 +126,8 @@ int is_same_word (const char *s, size_t l, const char *word) {
   return s && word && strlen (word) == l && !memcmp (s, word, l);
 }
 
+void fail_interface (struct tgl_state *TLS, struct in_ev *ev, int error_code, const char *format, ...) __attribute__ (( format (printf, 4, 5)));
+
 static void skip_wspc (void) {
   while (*line_ptr && ((unsigned char)*line_ptr) <= ' ') {
     line_ptr ++;
@@ -1140,9 +1142,7 @@ void do_send_typing (struct command *command, int arg_num, struct arg args[], st
   enum tgl_typing_status status = tgl_typing_typing; //de
   if (args[1].num != NOT_FOUND) {
     if (args[1].num > 0 && args[1].num > 10) {
-      TLS->error_code = ENOSYS;
-      TLS->error = strdup("illegal typing status");
-      print_fail(ev);
+      fail_interface (TLS, ev, ENOSYS, "illegal typing status");
       return;
     }
     status = (enum tgl_typing_status) args[1].num;  // if the status parameter is given, and is in range.