libutil/ev_zmq: handle EV_ERROR on io watcher

Problem: ev_zmq ignores events on its internal io watcher,
so when libev raises EV_ERROR on it because something went wrong
internally, nothing happens.

The internal io watcher is only used for its side effect of
unblocking the reactor when pollevents edge triggers from
"no events" to "some events".  The prep/check/idle watchers
do the heavy lifting.

Add a check for pending EV_ERROR events on the io watcher in
the check callback.  If found, notify the user via the ev_zmq
watcher callback.

This tracks a similar change in libflux/ev_flux.c, based on
experience with an internal failure documented in #2554.

src/common/libutil/ev_zmq.c

@@ -64,6 +64,9 @@ static void check_cb (struct ev_loop *loop, ev_check *w, int revents)
 
     if (handle == NULL)
         zw->cb (loop, zw, EV_ERROR);
+    else if (ev_is_pending (&zw->io_w)
+            && ev_clear_pending (loop, &zw->io_w) & EV_ERROR)
+        zw->cb (loop, zw, EV_ERROR);
     else if (zmq_getsockopt (handle, ZMQ_EVENTS, &zevents, &zevents_size) < 0)
         zw->cb (loop, zw, EV_ERROR);
     else if ((revents = ztoe (zevents) & zw->events))