LBSolrClient should limit size of Exception

LBSolrClient.request can return full list of zombie server in Exception.

See LBSolrClient.request.request :

    throw new SolrServerException(
        "No live SolrServers available to handle this request:" + zombieServers.keySet(), ex);

When a solr node is down, the list of zombie server can grow large because it can hold all the replicas hosted by the node.

Building this list can therefore becomes costly.

Since the full list of zombie server is typically not useful it would be enough to return the size of the zombieServers Map