Add clearer exceptions, warnings and docs

MediaBrowser.Controller/Entities/Folder.cs

@@ -1152,6 +1152,11 @@ namespace MediaBrowser.Controller.Entities
 
         public List<BaseItem> GetChildren(User user, bool includeLinkedChildren)
         {
+            if (user == null)
+            {
+                throw new ArgumentNullException(nameof(user));
+            }
+
             return GetChildren(user, includeLinkedChildren, null);
         }
 
@@ -1163,7 +1168,10 @@ namespace MediaBrowser.Controller.Entities
             }
 
             //the true root should return our users root folder children
-            if (IsPhysicalRoot) return LibraryManager.GetUserRootFolder().GetChildren(user, includeLinkedChildren);
+            if (IsPhysicalRoot)
+            {
+                return LibraryManager.GetUserRootFolder().GetChildren(user, includeLinkedChildren);
+            }
 
             var result = new Dictionary<Guid, BaseItem>();