Dislay an error if an exception occurs in the error handler

Related to #1598

application/Utils.php

@@ -463,3 +463,12 @@ function t($text, $nText = '', $nb = 1, $domain = 'shaarli')
 {
     return dn__($domain, $text, $nText, $nb);
 }
+
+/**
+ * Converts an exception into a printable stack trace string.
+ */
+function exception2text(Throwable $e): string
+{
+    return $e->getMessage() . PHP_EOL . $e->getFile() . $e->getLine() . PHP_EOL . $e->getTraceAsString();
+}
+

application/front/controller/visitor/ErrorController.php

@@ -28,10 +28,7 @@ public function __invoke(Request $request, Response $response, \Throwable $throw
             // Internal error (any other Throwable)
             if ($this->container->conf->get('dev.debug', false)) {
                 $this->assignView('message', $throwable->getMessage());
-                $this->assignView(
+                $this->assignView('stacktrace', exception2text($throwable));
-                    'stacktrace',
-                    nl2br(get_class($throwable) .': '. PHP_EOL . $throwable->getTraceAsString())
-                );
             } else {
                 $this->assignView('message', t('An unexpected error occurred.'));
             }

index.php

@@ -151,6 +151,12 @@
     $this->get('/history', '\Shaarli\Api\Controllers\HistoryController:getHistory')->setName('getHistory');
 })->add('\Shaarli\Api\ApiMiddleware');
 
+try {
     $response = $app->run(true);
-
     $app->respond($response);
+} catch (Throwable $e) {
+    die(nl2br(
+        'An unexpected error happened, and the error template could not be displayed.' . PHP_EOL . PHP_EOL .
+       exception2text($e)
+    ));
+}