moved error token/message from translator to rpn list class

the RpnList class now holds the error token and error message; the
associated accessor functions were also moved with an addition has error
function that returns if an error is present (the RPN list is cleared
when there is an error)

this change is needed so that when the RPN list is passed to the program
model, it also has the error information so that this can be displayed
in the edit box and program view widgets