Customise the Validation Error via a closure or string return

[peterfox]

I'd suggest that event validation has a callable to be able to fail a validation rather than the boolean method, although both of them can be available.

e.g.

class UserDroppedGoal extends Event
{
    public function validate(UserState $state, \Closure $fail): void
    {
        if (true) {
            $fail('The state isn't in the right mindset to run today, try again tomorrow?');
            return;
        }
    }
}

Or make it possible to return a string for the error:

class UserDroppedGoal extends Event
{
    public function validate(UserState $state, \Closure $fail): string|true
    {
        if (true) {
            return 'The state isn't in the right mindset to run today, try again tomorrow?';
        }
    }
}

I'd be happy to work on this if it's wanted and I can find the time.