Good OO design pattern for variant return type needs

I’ve got a validation function something like this (pseudo-code):

public function validate(Thing thing) {
    if (thing.a != "a") {
        return false; // most of the time
    if (thing.b != "b") {
        return false; // most of the time
    return true;

This function is in a domain service class. It’s called from lots of controller classes. 90% of the time, all I want is a boolean answer to “is thing valid?” But 10% of the time, I want to display a message on the UI as to why it’s invalid, if so. So instead of false when thing.a != "a" occurs, I want a string that says "thing's a is not a" or whatever.

I can think of several ways to manage this, but none of them make me particularly happy.

  1. Just return the string all the time and in the true case just return ok or something similar, but that feels fragile. It also makes the calling code have to do a string compare everywhere.
  2. Return an enumerated type, and have a helper method available to translate the enum into the string for when it’s needed, but that feels like I’ve just hidden the problem. And the calling code has to check a constant — better than 1, but not by much.
  3. Store the last message in a class property, to be obtained via a getter after a call to validate returns a false. That makes most of my calling code much cleaner since it only deals with boolean results except when it needs to know the cause. But is this fragile? It’s a web app, so it’s single threaded per request, so race conditions shouldn’t be a problem.

I feel like this is a common enough problem that there should be an elegant design pattern for it, but I’ve been unable to find it.