improve i/o error handling in ddfs_util:concatenate
improve i/o error handling in ddfs_util:concatenate

beni55/disco,seabirdzh/disco,ErikDubbelboer/disco,ErikDubbelboer/disco,pombredanne/disco,seabirdzh/disco,scrapinghub/disco,scrapinghub/disco,discoproject/disco,ktkt2009/disco,mozilla/disco,pombredanne/disco,ktkt2009/disco,scrapinghub/disco,simudream/disco,ErikDubbelboer/disco,mozilla/disco,beni55/disco,simudream/disco,simudream/disco,pavlobaron/disco_playground,simudream/disco,mwilliams3/disco,pooya/disco,pavlobaron/disco_playground,pombredanne/disco,ktkt2009/disco,seabirdzh/disco,discoproject/disco,seabirdzh/disco,discoproject/disco,oldmantaiter/disco,seabirdzh/disco,pooya/disco,ErikDubbelboer/disco,mozilla/disco,oldmantaiter/disco,simudream/disco,pombredanne/disco,ktkt2009/disco,mwilliams3/disco,ErikDubbelboer/disco,oldmantaiter/disco,beni55/disco,ktkt2009/disco,pooya/disco,scrapinghub/disco,mwilliams3/disco,mozilla/disco,oldmantaiter/disco,pavlobaron/disco_playground,discoproject/disco,mwilliams3/disco,pombredanne/disco,oldmantaiter/disco,mwilliams3/disco,pooya/disco,discoproject/disco,beni55/disco,beni55/disco,pavlobaron/disco_playground