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Account for fb2 books without covers

lector/models.py

@@ -14,11 +14,14 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+import logging
 import pathlib
 
 from PyQt5 import QtCore, QtWidgets
 from lector.resources import pie_chart
 
+logger = logging.getLogger(__name__)
+
 
 class BookmarkProxyModel(QtCore.QSortFilterProxyModel):
     def __init__(self, parent=None):