refactor(topologies): pass errors back from selectServer

The legacy topology types sometimes return an error, or a null
server to indicate selection failed.

Author: mbroadst

lib/core/topologies/mongos.js

@@ -1130,6 +1130,11 @@ Mongos.prototype.selectServer = function(selector, options, callback) {
   options = options || {};
 
   const server = pickProxy(this, options.session);
+  if (server == null) {
+    callback(new MongoError('server selection failed'));
+    return;
+  }
+
   if (this.s.debug) this.emit('pickedServer', null, server);
   callback(null, server);
 };

lib/core/topologies/replset.js

@@ -1111,6 +1111,16 @@ ReplSet.prototype.selectServer = function(selector, options, callback) {
   }
 
   const server = this.s.replicaSetState.pickServer(readPreference);
+  if (server == null) {
+    callback(new MongoError('server selection failed'));
+    return;
+  }
+
+  if (!(server instanceof Server)) {
+    callback(server, null);
+    return;
+  }
+
   if (this.s.debug) this.emit('pickedServer', options.readPreference, server);
   callback(null, server);
 };