[S02] Minor clarification

Make it clear that a specific type (C<Failure>) is returned instead of
something representing a general concept (failure).

S02-bits.pod

@@ -2847,8 +2847,8 @@ user-defined variable from you.  In fact, that's the default, and a
 lexical variable must have the trait "C<is dynamic>" to be
 visible via C<CALLER>.  (C<$_>, C<$!> and C<$/> are always
 dynamic, as are any variables whose declared names contain a C<*> twigil.)
-If the variable is not visible in the caller, it returns
-failure.  Variables whose names are visible at the point of the call but that
+If the variable is not visible in the caller, it returns C<Failure>.
+Variables whose names are visible at the point of the call but that
 come from outside that lexical scope are controlled by the scope
 in which they were originally declared as dynamic.
 Hence the visibility of C<< CALLER::<$*foo> >> is determined where