How to restore complete database keeping the site in maintenance mode in Drupal 7
Add a maintenance-page--offline.tpl.php page to your theme , and whenever the connection to DB is lost drupal will show this page instead of an error , but designing this page you cant use formal variables which are used in theme like $title or $styles and ... cause these are stored in DB and in your situation there is no access to that , so you must add them using php or just implement some strings . 

Check this page for more help : Theming the Drupal maintenance page