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.

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

Remove CFileSystemGCWii::Read32

A function that does the same thing was added to IVolume in 4cd00e3.