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

src/Exceptions/ApiException.cs

@@ -2,9 +2,9 @@ using System;
 
 namespace Novaloop.PaymoApi.Exceptions
 {
-    public class PaymoApiException : Exception
+    public class ApiException : Exception
     {
-        public PaymoApiException(int statusCode, string message) : base($"[{statusCode}]: {message})")
+        public ApiException(int statusCode, string message) : base($"[{statusCode}]: {message})")
         {
             StatusCode = statusCode;
         }