add not woeking error message on non-unique slug

public/index.php

@@ -85,6 +85,8 @@ $app->map(['GET', 'POST'], '/blog/create', function (Request $request, Response
     if (isset($_SESSION['username'])) {
         if ($request->getMethod() == 'GET') {
             addNavbar($response);
+            $response->getBody()->write($_SESSION['error']);
+            $response->getBody()->write('<hr/>');
             $response->getBody()->write('<form action="/blog/create" method="POST">');
             $response->getBody()->write('<label for="slug">slug:</label>');
             $response->getBody()->write('<input type="text" name="slug"/><br/>');
@@ -94,12 +96,20 @@ $app->map(['GET', 'POST'], '/blog/create', function (Request $request, Response
             $response->getBody()->write('<textarea name="content" rows="4" columns="50"></textarea>');
             $response->getBody()->write('<input type="submit"/>');
             addFooter($response);
+            unset($_SESSION['error']);
         } else {
             $data = $request->getParsedBody();
             global $db;
             $insertQuery = "INSERT INTO blogs (slug, title, content) VALUES ('" . $data['slug'] . "', '"  . $data['title'] . "', '"  . $data['content'] ."')";
-            $db->exec($insertQuery);
-            return $response->withHeader('Location', '/')->withStatus(302);
+            $ret = $db->exec($insertQuery);
+
+            if($ret) {
+                return $response->withHeader('Location', '/')->withStatus(302);
+            } else {
+                $err = $db->lastErrorMsg();
+                $_SESSION['error'] = $err;
+                return $response->withHeader('Location', '/blog/create')->withStatus(302);
+            }
         }
     } else {
         addNavbar($response);