Return "Iterator<Result<Event>>" instead of "Result<Iterator<Event>>" for Docker::events()

[flaviut]

If there's an error in initialization, it can be returned as the first (& only) element of the iterator. The current behaviour doesn't propagate any errors that occur while iterating the stream, it panics instead.

[softprops]

there are some updates on the way to return async streams - https://github.com/softprops/shiplift/pull/128/files#diff-b4aea3e418ccdb71239b96952d9cddb6R730