Add error propagation together with more error types

[tomgroenwoldt]

Hey! First of all, thanks for this crate and all your work for it :)

In my use case it's crucial to allow individual task errors. This crate already provides the Output enum for task outputs but only uses the variant Output::Err(String) for logging. I would really like the ability to propagate the contained string value as well as the task identifier all the way to the start function. This way, a user knows failed tasks and can work with that information in his code. What do you think of this idea? I'd be happy to work on this issue.

Thanks again!