Rescues and moves on to the next registered listener if one of the lister fails

I dont know the thinking behind not doing this. Maybe you expect the Listeners to take care of failures. But this is very useful for me . I have just added `rescue nil` for the moment. If you are interested in this direction, I can add a `:on_failure` option to subscribe which will accept a method name which is called on the listener. For example

```
class Listener
  def do_something
   ...
  end

  def log
    ...
  end
end

publisher.subscribe(Listener.new, on_failure: :log) #calls log method on failure of do_something
```