Print warning instead of error if not all judges are assigned

This is OK if some judges will be entered via the web appplication.

Fixes #307.

task #307: How handle some judges entered in webapp 
http://github.com/jpschewe/fll-sw/issues/issue/307
Add workflow for assign judges
Update training documentation

Fixes #308
Merge branch 'task_307-How_handle_some_judges_entered_offline'
Merge branch 'task_308-Update_training_docs'