[PATCH] Add ability for can_write to indicate why the file is not writable

[Rafael Espíndola]

> * Don't use can_write.
> * Change both exists and can_write to "std::error_code access(const
> Twine &Path, SomeEnum Mode)".
> * Change can_write to "std::error_code write_access(const Twine &Path)".

There is also can_write. OK, I will give it a try at merging these.

Cheers,
Rafael