improve error handling in workflow stepper

we recently merged https://github.com/galaxyproject/brc-analytics/pull/680

this at least tells ppl when something has gone awry which is good. id like to not block ppl though, when things go less than perfectly. ideally wed produce a warning rather than an error when a workflow stepper step didnt find data it was expecting or similar, maybe tell ppl theyll need to provide that thing themselves if theyd like to continue and direct them to contact us. then it should let ppl skip that step, populate the others and go to galaxy w what data they do manage to put together.