Fixes and sample

Packages/com.voidbotz.fonepayunity/Runtime/Models/FonepayError.cs

@@ -5,11 +5,18 @@ namespace Darkmatter.Fonepay
     public sealed class FonepayError : Exception
     {
         public int ErrorCode { get; }
+        public string Docs { get; }
 
-        public FonepayError(int errorCode, string message,
-            string docs)
+        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})";
         }
     }
 }