Fix lightning implementation, docs and tests

BTCPayServer.Client/GreenFieldAPIException.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Xml.Linq;
+
+namespace BTCPayServer.Client
+{
+    public class GreenFieldAPIException : Exception
+    {
+        public GreenFieldAPIException(Models.GreenfieldAPIError error):base(error.Message)
+        {
+            if (error == null)
+                throw new ArgumentNullException(nameof(error));
+            APIError = error;
+        }
+        public Models.GreenfieldAPIError APIError { get; }
+    }
+}