test_short_dio_read.c: add proper error handling

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
qa/workunits/direct_io/test_sync_io.c: add proper error handling

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
client/Client.cc: handle error if _lookup() fails

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>