Retwis ported to modern Redis data types.

The example (and related documentation at redis.io) was not updated for
5 years. Redis had no sorted sets and hashes when the original code was
written (!). An update was really needed.

We also use a modern PHP client now: Predis. A copy is shipped within
this repository to make life easier to newcomers trying Redis for the
first time via this example.

Predis/ServerException.php

@@ -0,0 +1,42 @@
+<?php
+
+/*
+ * This file is part of the Predis package.
+ *
+ * (c) Daniele Alessandri <suppakilla@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Predis;
+
+/**
+ * Exception class that identifies server-side Redis errors.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
+class ServerException extends PredisException implements ResponseErrorInterface
+{
+    /**
+     * Gets the type of the error returned by Redis.
+     *
+     * @return string
+     */
+    public function getErrorType()
+    {
+        list($errorType, ) = explode(' ', $this->getMessage(), 2);
+
+        return $errorType;
+    }
+
+    /**
+     * Converts the exception to an instance of ResponseError.
+     *
+     * @return ResponseError
+     */
+    public function toResponseError()
+    {
+        return new ResponseError($this->getMessage());
+    }
+}