Preprocessing filesets in coffea 0.7.x

[ikrommyd]

Is it possible to have some type of fileset preprocessing for coffea 0.7.x that sort of checks if a file is available and readable?
Should come in handy as a #1036 enhancement since the preprocessing part was removed due to its dependance on dasky things.

[valsdav]

I think that if the preprocessing in 0.7 breaks down because a file is not available, one can just blacklist a site and re-run the dataset tools to refresh the list of replicas. This approach has been a bit manual but successful in 0.7.x until now :)

Being able to get a report of the failed files from the preprocessing would be indeed good, as usually it's difficult to understand which file is failing..