Exercise 4.52: Implement a new construct called
if-fail that permits the user to catch the failure of an expression.
If-fail takes two expressions.  It evaluates the first expression as
usual and returns as usual if the evaluation succeeds.  If the evaluation
fails, however, the value of the second expression is returned, as in the
following example:


;;; Amb-Eval input:
(if-fail 
 (let ((x (an-element-of '(1 3 5))))
   (require (even? x))
   x)
 'all-odd)

;;; Starting a new problem
;;; Amb-Eval value:
all-odd

;;; Amb-Eval input:
(if-fail
 (let ((x (an-element-of '(1 3 5 8))))
   (require (even? x))
   x)
 'all-odd)

;;; Starting a new problem
;;; Amb-Eval value:
8