svcbackend: properly log error when supervisord can not be started

Output is already captured on SlapPopen instances and the output
is available with output attribute.
17 jobs for fix/proxy_destroy_partition in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

failed SlapOS.Eggs.UnitTest-Master.Python3

03:37:17

passed SlapOS.Eggs.UnitTest-Master.Python2

00:23:34

passed SlapOS.Eggs.UnitTest-Master.Python2

01:20:22

failed SlapOS.Eggs.UnitTest-Master.Python2

00:37:48

passed SlapOS.Eggs.UnitTest-Master.Python2

00:36:02

passed SlapOS.Eggs.UnitTest-Master.Python2

00:39:23

passed SlapOS.Eggs.UnitTest-Master.Python2

00:23:01

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python2

00:28:11

passed SlapOS.Eggs.UnitTest-Master.Python2

passed SlapOS.Eggs.UnitTest-Master.Python3

00:20:17

passed SlapOS.Eggs.UnitTest-Master.Python3

00:29:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:16:47

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:41

failed SlapOS.Eggs.UnitTest-Master.Python3

00:19:36

passed SlapOS.Eggs.UnitTest-Master.Python3

00:11:24