cluster: set proper error if we fail to locate a node.

This could happen if read preferences prevent finding a suitable node.

Author: chergert

src/mongoc/mongoc-cluster.c

@@ -775,6 +775,17 @@ _mongoc_cluster_select (mongoc_cluster_t             *cluster,
       }
    }
 
+   /*
+    * Mark the error as unable to locate a target node.
+    */
+   if (!count) {
+      bson_set_error (error,
+                      MONGOC_ERROR_CLIENT,
+                      MONGOC_ERROR_CLIENT_NO_ACCEPTABLE_PEER,
+                      "Failed to locate a suitable MongoDB node.");
+      RETURN (NULL);
+   }
+
    /*
     * Choose a cluster node within threshold at random.
     */