safe guarding MessageIdPlugin (#12)

Protect from invalid Func value registered with the plugin
Provide a meaningful stack trace when message id generator throws an exception

src/Microsoft.Azure.ServiceBus.MessageId/Guard.cs

@@ -0,0 +1,25 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Microsoft.Azure.ServiceBus.MessageId
+{
+    using System;
+
+    internal static class Guard
+    {
+        public static void AgainstEmpty(string argumentName, string value)
+        {
+            if (value != null && string.IsNullOrWhiteSpace(value))
+            {
+                throw new ArgumentNullException(argumentName);
+            }
+        }
+        public static void AgainstNull(string argumentName, object value)
+        {
+            if (value == null)
+            {
+                throw new ArgumentNullException(argumentName);
+            }
+        }
+    }
+}

src/Microsoft.Azure.ServiceBus.MessageId/MessageIdPlugin.cs

@@ -29,7 +29,23 @@ namespace Microsoft.Azure.ServiceBus.MessageId
         /// <param name="messageIdGenerator">Message ID generator to use.</param>
         public MessageIdPlugin(Func<Message, string> messageIdGenerator)
         {
-            this.messageIdGenerator = messageIdGenerator;
+            Guard.AgainstNull(nameof(messageIdGenerator), messageIdGenerator);
+            this.messageIdGenerator = SafeMessageIdGenerator(messageIdGenerator);
+        }
+
+        private Func<Message, string> SafeMessageIdGenerator(Func<Message, string> originalMessageIdGenerator)
+        {
+            return message =>
+            {
+                try
+                {
+                    return originalMessageIdGenerator(message);
+                }
+                catch (Exception exception)
+                {
+                    throw new Exception("An exception occurred when executing message ID generator Func", exception);
+                }
+            };
         }
 
         /// <summary>