Add a better message before installation when the current folder is not writable

C'est juste pour faire plus propre, chez moi je n'avais pas les bonnes permissions et me suis retrouvé avec une erreur et une exeception : 

    Warning: mkdir() [function.mkdir]: Permission denied in /var/www/local/ddb-rev/inc/rain.tpl.class.php on line 318

    Fatal error: Uncaught exception 'RainTpl_Exception' with message 'Cache directory cache/tpl/doesn't have write permission. Set write permission or set RAINTPL_CHECK_TEMPLATE_UPDATE to false. More details on http://www.raintpl.com/Documentation/Documentation-for-PHP-developers/Configuration/' in /var/www/local/ddb-rev/inc/rain.tpl.class.php:321 Stack trace: #0 /var/www/local/ddb-rev/inc/rain.tpl.class.php(274): RainTPL->compileFile('install', NULL, 'tpl/install.htm...', 'cache/tpl/', 'cache/tpl/insta...') #1 /var/www/local/ddb-rev/inc/rain.tpl.class.php(164): RainTPL->check_template('install') #2 /var/www/local/ddb-rev/install.php(184): RainTPL->draw('install') #3 {main} thrown in /var/www/local/ddb-rev/inc/rain.tpl.class.php on line 321