Improve "worker list" messages
Improve "worker list" messages

Now tells users if there are no workers to list rather than saying
failed to get workers.

johnmcdonnell/psiTurk,kachergis/psiTurk,andyh616/psiTurk,johnmcdonnell/psiTurk,berkeley-cocosci/psiTurk,NYUCCL/psiTurk,michaelpacer/psiTurk,rbtrace/psiTurk,suchow/psiTurk,mshvartsman/psiTurk,kachergis/psiTurk,kachergis/psiTurk,NYUCCL/psiTurk,pfeyz/psiTurk,berkeley-cocosci/psiTurk,suchow/psiTurk,johnmcdonnell/psiTurk,mshvartsman/psiTurk,rbtrace/psiTurk,mshvartsman/psiTurk,michaelpacer/psiTurk,suchow/psiTurk,NYUCCL/psiTurk,andyh616/psiTurk,pfeyz/psiTurk,rbtrace/psiTurk,pfeyz/psiTurk,berkeley-cocosci/psiTurk,andyh616/psiTurk,michaelpacer/psiTurk