Improve handling mkdir failure conditions
Following on from 4.8.1 and 4.8.3, throw Kohana_Exception if mkdir does not return TRUE for the occasions where mkdir doesn't succeed due to permissions but does not trigger a warning
Following on from 4.8.1 and 4.8.3, throw Kohana_Exception if mkdir does not return TRUE for the occasions where mkdir doesn't succeed due to permissions but does not trigger a warning