• Return non-zero status on write failuer.

Originally check the xo_finish return value can achive the POSIX standard. Howeve I use (trap "" PIPE; vmstat 2>stderr; echo $? >status | true to check this, it didn't work. I find check xo_finish return value is not enough here. Becuase xo_finish will always return 0, there is already a xo_flush call before xo_finish. So we need check xo_flush return value here.