input: distinguish an unknown escape sequence from an unbound key

It is misleading to say "Unknown Command" when there is no way to rebind
the relevant key stroke do anything useful.

src/winio.c

@@ -1143,7 +1143,9 @@ int parse_escape_sequence(WINDOW *win, int kbinput)
     /* If we got an unrecognized escape sequence, notify the user. */
     if (retval == ERR) {
 	if (win == edit) {
-	    statusbar(_("Unknown Command"));
+	    /* TRANSLATORS: This refers to a sequence of escape codes
+	     * (from the keyboard) that nano does not know about. */
+	    statusbar(_("Unknown sequence"));
 	    reset_cursor();
 	    curs_set(1);
 	    beep();