Added docs and test

Packages/com.darkmattergameproduction.com.darkmatter.fonepay-unity/Runtime/Models/FonepayError.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace Darkmatter.Fonepay
+{
+    public sealed class FonepayError : Exception
+    {
+        public int ErrorCode { get; }
+        public string Docs { get; }
+
+        public FonepayError(int errorCode, string message, string docs)
+            : base(message)
+        {
+            ErrorCode = errorCode;
+            Docs = docs ?? throw new ArgumentNullException(nameof(docs));
+        }
+
+        public override string ToString()
+        {
+            return $"{base.ToString()} (ErrorCode: {ErrorCode}, Docs: {Docs})";
+        }
+    }
+}