Simplify way to get failed steps

[flosell]

Follow up from #154:

It's a bit of an involved process getting information about failed steps, e.g. for slack notifications (https://gist.github.com/flosell/ff9976e9744814d60fcbb7d06f488f71). Maybe there should be code in LambdaCD itself to simplify this for users writing add-ons?

[thilo11]

+1 I was facing similar challenges in lambdacd-junit. It would be great to have hooks maybe similar to those in GoCD?

[flosell]

Thanks for the feedback @thilo11, however, I feel this is a slightly different use-case: What I meant with this issue was a way to get all failed steps in a pipeline when we get events via the event bus.

Your approach seems to be something that could be implemented with always-chaining which chains steps together inside a step and doesn't stop on failures. Even if inside the chain should be supported, giving you something that should be equivalent to GoCDs hooks:
https://github.com/flosell/lambdacd/blob/master/example/clj/todopipeline/steps.clj#L168-L174