Possibility to return an error immediately from ping handler if no searcher is available

Especially in a load-balanced environment it would be useful if it was possible to configure PingRequestHandler to return right away with an error status when a searcher is not (yet) available. This would allow the load-balancer to quickly fail over to a Solr instance that's able to serve the requests.

Currently the ping handler waits for the searcher to become available, which means the load-balancer has to either keep waiting or use a suitably short timeout, which is difficult to define in a way that provides a timely failover without false negatives.