Fix generate thumbnail task messages, don't load thumbnails when the cache file has been deleted

cps/helper.py

@@ -536,9 +536,13 @@ def delete_book(book, calibrepath, book_format):
 def get_thumbnails_for_books(books):
     books_with_covers = list(filter(lambda b: b.has_cover, books))
     book_ids = list(map(lambda b: b.id, books_with_covers))
+    cache = fs.FileSystem()
+    thumbnail_files = cache.list_cache_files(fs.CACHE_TYPE_THUMBNAILS)
+
     return ub.session\
         .query(ub.Thumbnail)\
         .filter(ub.Thumbnail.book_id.in_(book_ids))\
+        .filter(ub.Thumbnail.filename.in_(thumbnail_files))\
         .filter(ub.Thumbnail.expiration > datetime.utcnow())\
         .all()
 

cps/tasks/thumbnail.py

@@ -75,7 +75,7 @@ class TaskGenerateCoverThumbnails(CalibreTask):
                     else:
                         self.create_book_thumbnail(book, resolution)
 
-                self.message(u'Generating cover thumbnail {0} of {1}'.format(i, count))
+                self.message = u'Generating cover thumbnail {0} of {1}'.format(i + 1, count)
                 self.progress = (1.0 / count) * i
 
         self._handleSuccess()