How can I negate the return-value of a process?

I'm looking for a simple, but cross-platform negate-process that negates the value a process returns. It should map 0 to some value != 0 and any value != 0 to 0, i.e. the following command should return "yes, nonexistingpath doesn't exist":

ls nonexistingpath | negate && echo "yes, nonexistingpath doesn't exist."

The ! - operator is great but unfortunately not shell-independent.