Define ValidationErrorResult in SpaServices; use it in MusicStore

Microsoft.AspNet.SpaServices/ValidationErrorResult.cs

@@ -0,0 +1,30 @@
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNet.Mvc;
+using Microsoft.AspNet.Mvc.ModelBinding;
+
+namespace Microsoft.AspNet.SpaServices
+{
+    public class ValidationErrorResult : ObjectResult {
+        public const int DefaultStatusCode = 400;
+
+        public ValidationErrorResult(ModelStateDictionary modelState, int errorStatusCode = DefaultStatusCode)
+            : base(CreateResultObject(modelState))
+        {
+            if (!modelState.IsValid) {
+                this.StatusCode = errorStatusCode;
+            }
+        }
+
+        private static IDictionary<string, IEnumerable<string>> CreateResultObject(ModelStateDictionary modelState)
+        {
+            if (modelState.IsValid) {
+                return null;
+            } else {
+                return modelState
+                    .Where(m => m.Value.Errors.Any())
+                    .ToDictionary(m => m.Key, m => m.Value.Errors.Select(me => me.ErrorMessage));
+            }
+        }
+    }
+}