doc-src/Tutorial/Ifexpr/valif

Added an exception handler and error msg.

consts valif :: ifex => (nat => bool) => bool
primrec
"valif (CIF b)    env = b"
"valif (VIF x)    env = env x"
"valif (IF b t e) env = (if valif b env then valif t env
                                        else valif e env)"