journal: various fixes to journal_file_read_object()
    
    This fixes a bunch of issues:
    
    pread() returns ssize_t, and returns errors in 'errno', handle that
    correctly.
    
    More importantly: it might incompletely read data in case we hit
    EOF. Check for that, and handle it.
    
    Finally, rename the function to journal_file_read_object_header(), since
    it really doesn't read full objects, but only their headers.
    
    Follow-up for: 117e21121e857b4b7d81949542e8dd257265970a