Explore better error handling for background processing

[nonprofittechy]

Can we handle this better - or explicitly catch background processing failures and give the user some alternative way to download what they made, or try w/o background processing?

Currently, use_previously_cached_files = True is a best-efforts, so someone might be able to reach the download screen even if something went wrong with background processing.

but this might require the user seeing an ugly error screen first and then trying again--unclear

[nonprofittechy]

See https://docassemble.org/docs/background.html#background_error_action might be part of the story. note that it can give you a list of the variables that the interview was looking for, in order, so similar to the debugging you get from clicking the source button on a screen