workflow_history attribute may not exist on bootstrapping (constructInstance),
which leads AttributeError exception, which could have further side-effects
as this is called during dynamic class initialisation.
Also, original code from CMFCore.WorkflowTool, from which this code is
derived for bootstrap reasons, catches KeyError (when history exists,
just not for considered workflow), so follow the same pattern.
Laslty, also catch IndexError in case the workflow history entry would
exist empty - which would be the sign of a broken workflow history.