Split API classes and interfaces

docs/api/MongoDB/ServerError.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace MongoDB;
+
+interface ServerError extends ServerResult
+{
+    /**
+     * @return integer Server error code
+     */
+    function getCode();
+
+    /**
+     * @return string Server error message
+     */
+    function getMessage();
+}