File adapter should throw an error when file does not exist

[oleiade]

As of today

This

fa = File('/tmp/coucou')

won't raise even the though the file does not exist. It will only do when the load() method is called over it. It can be quite problematic, and not user-friendly.

[oleiade]

What I propose to fix that, is to pass an opened file descriptor to the File adapter instead. That way responsibilities of the file opening and possible raised errors catching are splitted.