translator: made severval minor improvements

changed done stack pop error token to return a token error instead of
the token modified for the error since a token error is what is needed
by callers of this function, and it is not necessary pop the item from
the done stack since the translator instance goes out of scope once the
error thrown

removed token set through function since it is no longer used
changed Qt forever with plain empty for
renamed expected error status function (to eliminate an abbreviation)
added 'noexcept' keyword on functions that don't throw exceptions
updated comments for changes