Fix a misuse of errno.

In menu_save_event, we checked ctx.error to see if an errno value had
been left in it by the savefile_write callback, but if so, then we
were passing the _current_ value of errno to strerror() in place of
the saved value in ctx.error.

This may well have been benign, but I spotted it in an eyeball review
just now and thought I'd better fix it before it bit anyone.

gtk.c

@@ -2287,7 +2287,7 @@ static void menu_save_event(GtkMenuItem *menuitem, gpointer data)
 	    if (ctx.error) {
 		char boxmsg[512];
 		sprintf(boxmsg, "Error writing save file: %.400s",
-			strerror(errno));
+			strerror(ctx.error));
 		error_box(fe->window, boxmsg);
 		goto free_and_return;
 	    }