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/gtkapp.c

@@ -109,6 +109,7 @@ https://wiki.gnome.org/Projects/GTK%2B/OSX/Bundling has some links.
 #define MAY_REFER_TO_GTK_IN_HEADERS
 
 #include "putty.h"
+#include "gtkmisc.h"
 
 char *x_get_default(const char *key) { return NULL; }
 
@@ -226,6 +227,19 @@ void new_app_win(GtkApplication *app)
     launch_new_session();
 }
 
+static void window_setup_error_callback(void *vctx, int result)
+{
+    g_application_release(G_APPLICATION(app));
+}
+
+void window_setup_error(const char *errmsg)
+{
+    create_message_box(NULL, "Error creating session window", errmsg,
+                       string_width("Some sort of fiddly error message that "
+                                    "might be technical"),
+                       TRUE, &buttons_ok, window_setup_error_callback, NULL);
+}
+
 static void activate(GApplication *app,
                      gpointer      user_data)
 {

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)
 {

unix/gtkwin.c

@@ -4639,17 +4639,21 @@ void new_session_window(Conf *conf, const char *geometry_string)
     inst->area = gtk_drawing_area_new();
     gtk_widget_set_name(GTK_WIDGET(inst->area), "drawing-area");
 
+    {
+        char *errmsg = setup_fonts_ucs(inst);
+        if (errmsg) {
+            window_setup_error(errmsg);
+            sfree(errmsg);
+            gtk_widget_destroy(inst->area);
+            sfree(inst);
+            return;
+        }
+    }
+
 #if GTK_CHECK_VERSION(2,0,0)
     inst->imc = gtk_im_multicontext_new();
 #endif
 
-    {
-        char *errmsg = setup_fonts_ucs(inst);
-        if (errmsg) {
-            fprintf(stderr, "%s: %s\n", appname, errmsg);
-            exit(1);
-        }
-    }
     inst->window = make_gtk_toplevel_window(inst);
     gtk_widget_set_name(GTK_WIDGET(inst->window), "top-level");
     {

unix/unix.h

@@ -135,6 +135,7 @@ void launch_duplicate_session(Conf *conf);
 void launch_new_session(void);
 void launch_saved_session(const char *str);
 void session_window_closed(void);
+void window_setup_error(const char *errmsg);
 #ifdef MAY_REFER_TO_GTK_IN_HEADERS
 GtkWidget *make_gtk_toplevel_window(void *frontend);
 #endif