translator: changed get expression to throw errors

removed return value
removed temporary creation of error tokens
removed done status enumerator since it is no longer used

changed detection of unary operator errors: when get expression finds an
unexpected unary operator, it throw an "expected binary operator or
end-of-statement" error; callers previously checked if the token was a
unary operator to report the appropriate error, but now that get
expression throws this error, no token is returned so now callers must
specifically check for this error and throw the appropriate error