Remove unused css: Prevent failure when book has invalid items marked as stylesheets
Check Book: Add checks for empty HTML/XML/CSS/image files in the book.  Fixes #1329971 [Private bug](https://bugs.launchpad.net/calibre/+bug/1329971)
Fix #1329777 [Translation DE](https://bugs.launchpad.net/calibre/+bug/1329777)