Considering JError was written to work with JError objects, I would hope it is safe to assume that something is indeed a JError object.  Otherwise, you start a precedent where you can't trust any code that isn't type hinted and have to check everything to make sure it's actually what you're expecting.  The switch to working with PHP's native exceptions did add a kink into this code, which is why I suggest checking that instead of checking if methods exist.