Revert "use more modern FALLBACK instead of .^add_fallback"

This Fallback mechanism seems not reliable in all cases.
Reverting this should make a5496e0 obsolete too.

This reverts commit 20c3ae3.

src/core/Failure.pm

@@ -37,12 +37,15 @@ my class Failure {
         self.exception.message ~ "\n" ~ self.backtrace;
     }
 
+    Failure.^add_fallback(
+        -> $, $ { True },
+        method ($name) {
+            $!exception.throw;
+        }
+    );
     method sink() {
         $!exception.throw($!backtrace) unless $!handled
     }
-    method FALLBACK(*@_) {
-        $!exception.throw;
-    }
 }
 
 proto sub fail(|) {*};