Moved error handler classes from Expressive namespace to ErrorHandler namespace

module/Common/src/ErrorHandler/ErrorHandlerInterface.php

@@ -0,0 +1,18 @@
+<?php
+namespace Shlinkio\Shlink\Common\ErrorHandler;
+
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Message\ResponseInterface as Response;
+
+interface ErrorHandlerInterface
+{
+    /**
+     * Final handler for an application.
+     *
+     * @param Request $request
+     * @param Response $response
+     * @param null|mixed $err
+     * @return Response
+     */
+    public function __invoke(Request $request, Response $response, $err = null);
+}