In /lib/setup.php after line 153 (echo '<p>Error: Database connection failed.</p>';) place:

//[MDL-12049][IJR] this fix resolves a rather awful moodle error msg
if(isset($CFG->dberrormsg)){
    echo '<p>'.$CFG->dberrormsg.'</p>';		
} else {
    echo '<p>It is possible that the database is overloaded or otherwise not running properly.</p>';
    echo '<p>The site administrator should also check that the database details have been correctly specified in config.php</p>';		
}


In /config.php place:
$CFG->dberrormsg = 'Database down message goes here.';