I'm experimenting with the Upload class and purposefully set the upload path to something nonexistent and set 'create_path' to false to see what happens when a file upload fails.
It looks like the code tries to throw an Exception in classes/upload.php line 446:
'Can\'t move the uploaded file. Destination path specified does not exist.'
However the error reported is actually:
ErrorException [ Error ]: Class 'Fuel\Core\Exception' not found
I'm not sure, but it looks like it's expecting to find an "Exception" class?