exceptions.cpp: Add log info

rt/test/exceptions.cpp

@@ -30,6 +30,7 @@ Join<void> join(int max) {
     v.push_back(co_await fork(fib()));
   }
   co_await fork(fail_join());
+  puts("info: fork did not throw");
   co_await Sync();
   for (auto & f : v) {
     printf("result = %d\n", f.get());