[FIX] auth_signup: properly display the error message

Was "Internal Error 500"

Introduced at c3bf161

This closes #20941

addons/auth_signup/controllers/main.py

@@ -46,7 +46,7 @@ class AuthSignupHome(Home):
                         ).send_mail(user_sudo.id, force_send=True)
                 return super(AuthSignupHome, self).web_login(*args, **kw)
             except UserError as e:
-                qcontext['error'] = e.message or e.name
+                qcontext['error'] = e.name or e.value
             except (SignupError, AssertionError) as e:
                 if request.env["res.users"].sudo().search([("login", "=", qcontext.get("login"))]):
                     qcontext["error"] = _("Another user is already registered using this email address.")