Add ApiSystemException class.

src/Exceptions/ApiInvalidParamException.php

@@ -2,8 +2,6 @@
 
 namespace PdToolKit\Exceptions;
 
-use Throwable;
-
 class ApiInvalidParamException extends ApiException
 {
     public function __construct($message, Throwable $previous = null)

src/Exceptions/ApiSystemException.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace PdToolKit\Exceptions;
+
+class ApiSystemException extends ApiException
+{
+    public function __construct($message = "网络波动，稍后再试", Throwable $previous = null)
+    {
+        parent::__construct($message, 400000, $previous);
+    }
+}