broadway: Quiet initialisation failures

Broadway is the only GTK+ backend that throws an error on stderr when
failing to initialise, which causes problems when gtk_init_check() is
used and unexpected error output is generated.

This causes hotdoc to fail when generating a GTK plugin's documentation
instead of failing quietly.

"Unable to init server: Could not connect: Connection refused"

gdk/broadway/gdkdisplay-broadway.c

@@ -213,7 +213,7 @@ _gdk_broadway_display_open (const char *display_name)
   broadway_display->server = _gdk_broadway_server_new (display, display_name, &error);
   if (broadway_display->server == NULL)
     {
-      g_printerr ("Unable to init Broadway server: %s\n", error->message);
+      g_debug ("Unable to init Broadway server: %s\n", error->message);
       g_error_free (error);
       return NULL;
     }