chore: refactoring some code to introduce a common base class for all api classes

src/Exceptions/ApiException.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace Novaloop.PaymoApi.Exceptions
+{
+    public class ApiException : Exception
+    {
+        public ApiException(int statusCode, string message) : base($"[{statusCode}]: {message})")
+        {
+            StatusCode = statusCode;
+        }
+
+        public int StatusCode { get; }
+    }
+}