add table designer handlers (#1269)

* add table designer handlers

* add comments

src/Microsoft.SqlTools.ServiceLayer/TableDesigner/Contracts/TableDesignerValidationError.cs

@@ -0,0 +1,27 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.Hosting.Protocol.Contracts;
+using Microsoft.SqlTools.ServiceLayer.Utility;
+using Microsoft.SqlTools.Utility;
+
+namespace Microsoft.SqlTools.ServiceLayer.TableDesigner.Contracts
+{
+    /// <summary>
+    /// Validation error
+    /// </summary>
+    public class TableDesignerValidationError
+    {
+        /// <summary>
+        /// The error message
+        /// </summary>
+        public string Message { get; set; }
+
+        /// <summary>
+        /// The property associated with the message, could be a string or TableDesignerPropertyIdentifier
+        /// </summary>
+        public object Property { get; set; }
+    }
+}