Add an errors signal to RACCommand

Based on [ReactiveAsyncCommand](https://github.com/reactiveui/ReactiveUI/blob/master/ReactiveUI.Xaml/ReactiveAsyncCommand.cs#L142) and some Campfire discussions, this adds a signal of `NSError`s to `RACCommand`, so that it becomes possible to uniformly handle errors that occur in a command's asynchronous work.

@alanjrogers @xpaulbettsx