Add proper Error Message for Error Cases #6
Description
Right now fulibWorkflows doesn't create helpful error messages if something wasn't modelled correctly in the yaml file.
Possible Error cases have to be evaluated and custom errors must be thrown at the correct places.
Additionaly note that it is a good idea or more than that. It is required to include the correct error generation for proper display in fulib.org and the codemirror used there.