Split up HandleErrors middleware into distinct classes

These are completely distinct functionalities, toggled through the
system-wide debug flag. By moving the selection of the middleware
to use to the place where the middleware pipe is built, we make
the middleware itself be unaware of these flags. The two classes
are more focused on what they are doing, with the constructor
dependencies clearly representing their requirements.

In addition, this means we can just use the HandleErrorsWithWhoops
middleware in the installer, which means we do not need to worry
about how to inject a SettingsRepositoryInterface implementation
when flarum is not yet set up.

src/Http/Middleware/HandleErrorsWithWhoops.php

@@ -0,0 +1,34 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Http\Middleware;
+
+use Exception;
+use Franzl\Middleware\Whoops\WhoopsRunner;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Server\MiddlewareInterface as Middleware;
+use Psr\Http\Server\RequestHandlerInterface as Handler;
+
+class HandleErrorsWithWhoops implements Middleware
+{
+    /**
+     * Catch all errors that happen during further middleware execution.
+     */
+    public function process(Request $request, Handler $handler): Response
+    {
+        try {
+            return $handler->handle($request);
+        } catch (Exception $e) {
+            return WhoopsRunner::handle($e, $request);
+        }
+    }
+}