Streamlines validation response handling in controllers

This makes validation response handling more inline with aspnetcore patterns.

src/Umbraco.Web.Common/Extensions/ModelStateExtensions.cs

@@ -0,0 +1,55 @@
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+
+namespace Umbraco.Extensions
+{
+    public static class ModelStateExtensions
+    {
+        /// <summary>
+        /// Checks if there are any model errors on any fields containing the prefix
+        /// </summary>
+        /// <param name="state"></param>
+        /// <param name="prefix"></param>
+        /// <returns></returns>
+        public static bool IsValid(this ModelStateDictionary state, string prefix) =>
+            state.Where(v => v.Key.StartsWith(prefix + ".")).All(v => !v.Value.Errors.Any());
+
+        public static IDictionary<string, object> ToErrorDictionary(this ModelStateDictionary modelState)
+        {
+            var modelStateError = new Dictionary<string, object>();
+            foreach (KeyValuePair<string, ModelStateEntry> keyModelStatePair in modelState)
+            {
+                var key = keyModelStatePair.Key;
+                ModelErrorCollection errors = keyModelStatePair.Value.Errors;
+                if (errors != null && errors.Count > 0)
+                {
+                    modelStateError.Add(key, errors.Select(error => error.ErrorMessage));
+                }
+            }
+            return modelStateError;
+        }
+
+        /// <summary>
+        /// Serializes the ModelState to JSON for JavaScript to interrogate the errors
+        /// </summary>
+        /// <param name="state"></param>
+        /// <returns></returns>
+        public static JsonResult ToJsonErrors(this ModelStateDictionary state) =>
+            new JsonResult(new
+            {
+                success = state.IsValid.ToString().ToLower(),
+                failureType = "ValidationError",
+                validationErrors = from e in state
+                                   where e.Value.Errors.Count > 0
+                                   select new
+                                   {
+                                       name = e.Key,
+                                       errors = e.Value.Errors.Select(x => x.ErrorMessage)
+                                           .Concat(
+                                               e.Value.Errors.Where(x => x.Exception != null).Select(x => x.Exception.Message))
+                                   }
+            });
+    }
+}