Catch the absence of $databases settings array early

The code looks nice; just need some testing. Oops, just found an issue with the _drupal_install_required function in your patch. The install_goto will not be called if it already is defined. The if condition isn't needed since PHP takes care of that naturally.

OT: I think a usability issue is one where the DB isn't available but has been installed already. Should the _drupal_install_required function check for !empty($databases) and put up a no_db.php instead?

 /**
  * Send the user to the installer page.
  */
  function _drupal_install_required() {
    global $databases;

    include_once DRUPAL_ROOT . '/includes/install.inc';

    if (empty($databases)) {
     install_goto('install.php');
    }
    else {
     install_goto('no_db.php');  
    }
  }

But since install.inc isn't normally included then install_goto will not be defined to give a clue to update versus install. Therefore the if condition will always be met except for during the install phase. Ok, I can see that the install phase will call this function as well and the condition is avoiding endless recursion. I still need to test this.

The last submitted patch failed testing.