Display an error message on failed IMAP logins

The redirect to the accounts URL was just confusing for users
that hit bugs with the authentication system. It's better to
be explicit about the situation.

plugins/sso/sso.php

@@ -75,7 +75,8 @@ class sso extends rcube_plugin
 
   function login_failed($args)
   {
-    header('Location: https://accounts.autistici.org/');
+    http_response_code(500);
+    echo('<h3>There has been a login error with the mail server. Please go back and try again.</h3>');
     exit;
   }