fix: properly handle err messages in MongoDB 2.6 servers

In server versions where we cannot assume we are always working
with commands, we need to encode the knowledge that we are running
a command to inform how we process the response

lib/cmap/connection.js

@@ -243,14 +243,16 @@ function messageHandler(conn) {
         conn.emit('clusterTimeReceived', document.$clusterTime);
       }
 
-      if (document.writeConcernError) {
-        callback(new MongoWriteConcernError(document.writeConcernError, document));
-        return;
-      }
+      if (operationDescription.command) {
+        if (document.writeConcernError) {
+          callback(new MongoWriteConcernError(document.writeConcernError, document));
+          return;
+        }
 
-      if (document.ok === 0 || document.$err || document.errmsg || document.code) {
-        callback(new MongoError(document));
-        return;
+        if (document.ok === 0 || document.$err || document.errmsg || document.code) {
+          callback(new MongoError(document));
+          return;
+        }
       }
     }
 
@@ -290,6 +292,7 @@ function write(command, options, callback) {
     fullResult: typeof options.fullResult === 'boolean' ? options.fullResult : false,
     noResponse: typeof options.noResponse === 'boolean' ? options.noResponse : false,
     documentsReturnedIn: options.documentsReturnedIn,
+    command: !!options.command,
 
     // for BSON parsing
     promoteLongs: typeof options.promoteLongs === 'boolean' ? options.promoteLongs : true,