Created content based error handler which allows managing errors in a different way depending on the Accepted content type from the client

module/Rest/src/Expressive/JsonErrorHandler.php

@@ -0,0 +1,39 @@
+<?php
+namespace Shlinkio\Shlink\Rest\Expressive;
+
+use Psr\Http\Message\RequestInterface as Request;
+use Psr\Http\Message\ResponseInterface as Response;
+use Shlinkio\Shlink\Common\Expressive\ErrorHandlerInterface;
+use Zend\Diactoros\Response\JsonResponse;
+
+class JsonErrorHandler implements 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)
+    {
+        $status = $response->getStatusCode();
+        $responsePhrase = $status < 400 ? 'Internal Server Error' : $response->getReasonPhrase();
+        $status = $status < 400 ? 500 : $status;
+
+        return new JsonResponse([
+            'error' => $this->responsePhraseToCode($responsePhrase),
+            'message' => $responsePhrase,
+        ], $status);
+    }
+
+    /**
+     * @param string $responsePhrase
+     * @return string
+     */
+    protected function responsePhraseToCode($responsePhrase)
+    {
+        return strtoupper(str_replace(' ', '_', $responsePhrase));
+    }
+}