added operand recreate function, error for no recreate function

instead of assuming that no recreate function indicates an operand
(since it could also mean that a recreate function is not implemented),
an operand recreate function was implemented (just pushes the string of
the token), which was added to the constant and variable code table
entries, and if there is no recreate function, the string of the token
(which may be empty) is surrounded by question marks to indicate an
error and pushed to the holding stack; and a check was also needed for
a valid code in the token before retrieving its recreate function