civicrm upgrade to 4.1 via drush ends in drush error

When running drush civicrm-upgrade-db, drush terminates unexpectedly.

That happens because the theme call made during the upgrade process calls exit.

The attached patch fixes this problem and doesn't seem to cause other problems. However, it makes a potentially impactful change in the theme function in CRM/Utils/System/Base.php.

I think it's the correct change, since the current behavior prints and exits even if the $print variable is set to false.

However, it would be good to get another set of eyes before committing.