gui: print a warning when we can't connect to the screen

Because the exception thrown is less than obvious.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

tuhi-gui.in

@@ -39,10 +39,14 @@ def gtk_style():
     }
     """
 
+    screen = Gdk.Screen.get_default()
+    if screen is None:
+        print('Error: Unable to connect to screen. Make sure DISPLAY or WAYLAND_DISPLAY are set', file=sys.stderr)
+        sys.exit(1)
     style_provider = Gtk.CssProvider()
     style_provider.load_from_data(css)
     Gtk.StyleContext.add_provider_for_screen(
-            Gdk.Screen.get_default(),
+            screen,
             style_provider,
             Gtk.STYLE_PROVIDER_PRIORITY_APPLICATION
             )