write more sensible error messages to console if items do not contain a 'class' key

sloth/annotations/model.py

@@ -519,7 +519,11 @@ class AnnotationModelItem(KeyValueModelItem):
     def data(self, role=Qt.DisplayRole, column=0):
         if role == Qt.DisplayRole:
             if column == 0:
-                return self['class']
+                try:
+                    return self['class']
+                except KeyError:
+                    LOG.error('Could not find key class in annotation item. Please check your label file.')
+                    return '<error - no class set>'
             elif column == 1 and self.isUnconfirmed():
                 return '[unconfirmed]'
             else:

sloth/gui/annotationscene.py

@@ -98,7 +98,11 @@ class AnnotationScene(QGraphicsScene):
             child = self._image_item.childAt(row)
             if not isinstance(child, AnnotationModelItem):
                 continue
-            label_class = child['class']
+            try:
+                label_class = child['class']
+            except KeyError:
+                LOG.error('Could not find key class in annotation item. Skipping this item. Please check your label file.')
+                continue
             item = self._itemfactory.create(label_class, child)
             if item is not None:
                 self.addItem(item)