Add test for message error. part of #269.

Author: yfakariya

test/MsgPack.UnitTest/Serialization/RegressionTests.cs

@@ -540,6 +540,12 @@ public void TestIssue269()
 			Assert.That( target.UnderlyingType, Is.EqualTo( typeof( MessagePackExtendedTypeObject ) ) );
 			Assert.That( target.IsTypeOf<byte[]>(), Is.False );
 			Assert.That( target.IsTypeOf<MessagePackExtendedTypeObject>(), Is.True );
+
+			var forBinary = Assert.Throws<InvalidOperationException>( () => target.AsBinary() );
+			Assert.That( forBinary.Message, Is.EqualTo( "Do not convert MsgPack.MessagePackExtendedTypeObject MessagePackObject to System.Byte[]." ) );
+
+			var forString = Assert.Throws<InvalidOperationException>( () => target.AsString() );
+			Assert.That( forString.Message, Is.EqualTo( "Do not convert MsgPack.MessagePackExtendedTypeObject MessagePackObject to System.String." ) );
 		}
 	}
 }