mapCube fails to show errors when ispar=true

[lazarusA]

let's try to debug and fix.

[felixcremer]

This is because we call pmap with on_error=identity. This was introduced by @meggart in #265. I am not sure, what is the use case to not just rethrow the error?
Do we have cases where we expect a failure at certain parts of the data but we would like the mapCube to still succeed? That would be something for the on_error keyword.