bkr task-details: Display a proper error message for non-existent task

Bug:1022411
Change-Id: Ia6833c86b6c2daa6f043c7241b05a0d9db153319
task_info['worker'] should be None instead of {}

Bug: 1066269
Change-Id: Ie68b532999043b6b3dfd24cd0a90e537ee07dd2e
Merge branch 'release-0.16' into develop