mainloop: handle more gracefully if fd is closed before delete_io_watch.

mainloop: handle more gracefully if fd is closed before delete_io_watch.

EPOLL_CTL_DEL sets errno to ENOENT if w->fd is closed (by the user)
after mrp_del_io_watch is called but before delete_io_watch gets around
to purge the watch and update our epoll bookkeeping. Don't treat this as
an error and avoid printing an error message.