continue-on-error in workflows, analogous to github actions

Hi guys :wave:

Big fan of weasel and used it to build a prototype recently - annotation and learning layout of PDFs (using prodigy, you're probably familiar :wink: )

One thing that came up for me: there's a workflow in which I would like to annotate data with prodigy and then directly use the data in a with `prodigy db-out` and transform it and train a model. However, the preferred way to shut down prodigy seems to be user interrupt (according to this [prodigy issue - how to quit prodigy](https://support.prodi.gy/t/alternative-exit-to-quit-in-terminal/569) )

So right now if a step in a weasel workflow has a non-zero exit status, the workflow doesn't continue and this is desired behavior if I understand correctly - however, in this case it would be really nice to tell weasel to continue the command on error like it's usually done in github actions, or better yet, ignore certain exit codes (like 130)