Enhance Server-side Error Message on TaskDef Variable Validation

[diablouma]

When we are trying to assign a value which type does not match with the defined in the WfSpec we throw this error:

Node 1-save-ds-cluster-on-db-TASK: Input variable 3 needs to be STR but cannot be

From the methid validate on the class TaskNodeModel:
"Input variable " + i + " needs to be " + taskDefVar.getType() + " but cannot be!");

instead of the index, try to log the variable name to improve the UX