add error message for captcha generation

app/controllers/welcome.js

@@ -43,6 +43,9 @@ class WelcomeController extends SiteController {
   }
 
   async getSignupCaptcha (req, res) {
+    if (!req.session || !req.session.captcha || !req.session.captcha.signup) {
+      return res.status(500).end('Session is not in a valid state for generating a captcha image');
+    }
     const signupCaptcha = captcha(req.session.captcha.signup, {
       color: false,
       noise: 3,