Merge branch 'master' of https://github.com/salfter/ciderpress into salfter-master
Merge branch 'salfter-master'
Merge branch 'master' of https://github.com/fadden/ciderpress
Better error message when extracting files

The file extraction dialog allows you to select file parts, so you
can choose to exclude resource forks or just extract disk images.  If
you don't choose any parts, nothing will extract, and you get a
confusingly generic message about nothing matching the criteria.

This adds a specific error message for the case where no parts are
selected.