Lacking a good design pattern for presenting errors
In the old days, you'd present errors using error dialogs. Nowadays, the HIG recommends avoiding those, but we doesn't particularly offer an alternative.
One potential solution here is in-app notifications (#22 (closed)), but they are only really good for presenting a single line message. What about cases where the error is necessarily longer? Or what about cases where there's technical output that we want to enable users to access?
There's some old design work which might be relevant.