Implement middleware for handling errors according to JSON API spec

src/Api/Middleware/JsonApiErrors.php

@@ -0,0 +1,36 @@
+<?php namespace Flarum\Api\Middleware;
+
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Zend\Diactoros\Response\StringResponse;
+use Zend\Stratigility\ErrorMiddlewareInterface;
+
+class JsonApiErrors implements ErrorMiddlewareInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function __invoke($error, Request $request, Response $response, callable $out = null)
+    {
+        $errorObject = [
+            'title' => $error->getMessage(),
+        ];
+
+        $status = 500;
+
+        // If it seems to be a valid HTTP status code, we pass on the
+        // exception's status.
+        $errorCode = $error->getCode();
+        if (is_int($errorCode) && $errorCode >= 400 && $errorCode < 600) {
+            $status = $errorCode;
+        }
+
+        // JSON API errors must be collected in an array under the
+        // "errors" key in the top level of the document
+        $data = [
+            'errors' => [$errorObject]
+        ];
+
+        return StringResponse::json($data, $status);
+    }
+}