failable convenience init?

import Foundation

public class Thing: NSObject {
    public enum State {
        case yes, no, maybe
    }
    
    public var state:State
    
    internal init() {
        self.state = State.maybe
    }
}

extension Thing {

    public convenience init(json:[String:AnyObject]) {
        self.init()
        self.populateFromJson(json)
    }
    
    public func populateFromJson(json:[String:AnyObject]) {
        // do stuff
    }
}


public class ThingModelB: Thing {
    public var foo:String
    
    public init(foo:String) {
        self.foo = foo
    }
}

extension ThingModelB {
    public convenience init(json:[String:AnyObject]) {
        if json["foo"] {
            let jsonFoo = json["uuid"]! as String
            self.init(foo: jsonFoo)
        }
        else {
            // how about, don't f'king init ?!
        }
    }
}