GTK: fix the exit(1) response to startup-time font errors.

For gtkapp-based tools that will have to stop being a program-fatal
error, so I've turned it into a function called window_setup_error
(which I could in principle reuse for other problems in the long and
tortuous progress of new_session_window), and kept the original
handling in gtkmain.c's implementation of that function while gtkapp.c
does something more sensible with a message box.

unix/gtkmain.c

@@ -309,6 +309,12 @@ void cmdline_error(const char *p, ...)
     exit(1);
 }
 
+void window_setup_error(const char *errmsg)
+{
+    fprintf(stderr, "%s: %s\n", appname, errmsg);
+    exit(1);
+}
+
 int do_cmdline(int argc, char **argv, int do_everything, int *allow_launch,
                Conf *conf)
 {