Improve IS_SSSD_ERROR() macro
    
    We need to mask the first part with 0xFFFF or there is a slight chance an
    unrelated error code would match even if the upper part is not exactly
    equal to ERR_BASE but just has all it's bits and some more.
    Also make the macro more reasable by adding another helper macro for filtering
    the base.
    Finally compare err and ERR_LAST directly w/o masking err, or the comparison
    will always return true.