[C++] Add an Error Handling section to the Authentication page (#98)

* add auth error handling

source/includes/authentication.cpp

@@ -45,4 +45,16 @@ auto client = mongocxx::client(uri);
 auto uri = mongocxx::uri("mongodb://<db_username>:<db_password>@<hostname>:<port>/?"
                          "authMechanism=PLAIN&tls=true");
 auto client = mongocxx::client(uri);
-// end-plain
+// end-plain
+
+// start-auth-err
+try {
+    auto uri = mongocxx::uri("<connection string>");
+    auto client = mongocxx::client(uri);
+
+    client["db"].run_command(<any command requiring authorization>);
+
+} catch (const mongocxx::exception& ex) {
+    std::cerr << "Error: " << ex.what() << std::endl;
+}
+// end-auth-err

source/security/authentication.txt

@@ -273,6 +273,23 @@ URI to ``"MONGODB-AWS"``, as shown in the following example:
    to retrieve credentials only from an EC2 instance, ensure that the
    environment variables are not set.
 
+Error Handling
+--------------
+
+You can handle authentication errors by catching ``mongocxx::exception``
+errors. Inspect the error message string by using the ``what()`` method, as
+shown in the following example:
+
+.. literalinclude:: /includes/authentication.cpp
+   :language: cpp
+   :copyable: true
+   :start-after: // start-auth-err
+   :end-before: // end-auth-err
+
+For more information, see `Operation Exceptions
+<{+api+}/topic-mongocxx-examples-operation-exceptions.html>`__ in the API
+documentation.
+
 API Documentation
 -----------------