When many surfaces are created, sway can run out of file descriptors,
making wl_event_loop_add_timer (which creates a timerfd) fail and
return NULL. This patch posts a "no memory" error when that is the case,
and only removes the timer if it was created.
(Why "no memory"? It is not easy to distinguish between failures due to
running out of memory and failures due to running out of file
descriptors. Also, using the newer `wl_client_post_implementation_error`
function would lead to an increased version requirement for the
libwayland-server dependency.)
