Fix lightning implementation, docs and tests

BTCPayServer.Client/Models/GreenfieldAPIError.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace BTCPayServer.Client.Models
+{
+    public class GreenfieldAPIError
+    {
+        public GreenfieldAPIError()
+        {
+
+        }
+        public GreenfieldAPIError(string code, string message)
+        {
+            if (code == null)
+                throw new ArgumentNullException(nameof(code));
+            if (message == null)
+                throw new ArgumentNullException(nameof(message));
+            Code = code;
+            Message = message;
+        }
+        public string Code { get; set; }
+        public string Message { get; set; }
+    }
+}