Callers can now check whether reads fail, either by checking the return
value or by setting the buffer to a known bad value and seeing if it stays
untouched. I've added error checks to FileSystemGCWii and Boot_BS2Emu,
but not to Boot since it doesn't check any of its other reads either.

Rather than rely on the developer to do the right thing,
just make the default behavior safely deallocate resources.

If shared semantics are ever needed in the future, the
constructor that takes a unique_ptr for shared_ptr can
be used.

IFileSystem::GetBootDOLSize acts the same. The only difference is that it
reads the DOL offset on its own, but this change makes that optional.

Filesystem: Return file list reference instead of modifying argument