Create default MqttRetainedMessagesManager in options

Source/MQTTnet/Exceptions/MqttConfigurationException.cs

@@ -0,0 +1,21 @@
+using System;
+
+namespace MQTTnet.Exceptions
+{
+    public class MqttConfigurationException : Exception
+    {
+        protected MqttConfigurationException()
+        {
+        }
+
+        public MqttConfigurationException(Exception innerException)
+            : base(innerException.Message, innerException)
+        {
+        }
+
+        public MqttConfigurationException(string message)
+            : base(message)
+        {
+        }
+    }
+}