Update Hash.php

I'm not sure on whether this is a problem with my local app but I have seen it a couple of times in a couple of projects: 
Warning (2): Invalid argument supplied for foreach() [CORE/Cake/Utility/Hash.php, line 52]

I think Hash::get should be able to handle this better rather than throwing an error in a core Util file.
I also think the additional exception might not be necessary
it can only be `@param string|array` and in other cases the notice/error will notify the developer of wrong doing.