VariableReferenceVertex: throw better exceptions

[mtrberzi]

   } else if (incoming.size() == 0) {
      // not assigned
      // TODO we would like to throw this exception but we need a Variable
      //throw new VariableNotAssignedException(id);
      throw new UndefinedBehaviourError("variable '" + id + "' not assigned");
    } else { 
      // multiply assigned
      // TODO we would like to throw this exception but we need a Variable
      //throw new MultipleAssignmentException(id);
      throw new UndefinedBehaviourError("variable '" + id + 
          "' multiply assigned");
    }