broker: improve LOST error message

Problem: many "transitioning to LOST due to EHOSTUNREACH error on send"
messages were logged during shutdown of a large instance.

This is still not well understood but we can perhaps get a little more
information for next time.

Add the previous state and the time the peer has spent in that
state (in whole seconds).

Hopefully will help with #5881 if it occurs again.

src/broker/overlay.c

@@ -777,9 +777,12 @@ static int overlay_sendmsg_child (struct overlay *ov, const flux_msg_t *msg)
             && (child = child_lookup_online (ov, uuid))) {
             flux_log (ov->h,
                       LOG_ERR,
-                      "%s (rank %d) transitioning to LOST due to %s",
+                      "%s (rank %d) transitioning to %s->LOST"
+                      " after %ds due to %s",
                       flux_get_hostbyrank (ov->h, child->rank),
                       (int)child->rank,
+                      subtree_status_str (child->status),
+                      (int)(monotime_since (child->status_timestamp) / 1000.0),
                       "EHOSTUNREACH error on send");
             overlay_child_status_update (ov, child, SUBTREE_STATUS_LOST);
         }