BlenderKit: fix upload reporting when packing fails

Now sends a clear message to the UI and cancels upload
also fixes a small bug in update_down_up

blenderkit/__init__.py

@@ -254,7 +254,7 @@ def udate_down_up(self, context):
     s = context.scene
     wm = bpy.context.window_manager
     props = s.blenderkitUI
-    if wm['search results'] == None and props.down_up == 'SEARCH':
+    if wm.get('search results') == None and props.down_up == 'SEARCH':
         search.search()
 
 def switch_search_results(self, context):

blenderkit/upload.py

@@ -986,6 +986,10 @@ class Uploader(threading.Thread):
                     "file_path": fpath
                 })
 
+                if not os.path.exists(fpath):
+                    self.send_message ("File packing failed, please try manual packing first")
+                    return {'CANCELLED'}
+
             self.send_message('Uploading files')
 
             uploaded = upload_bg.upload_files(self.upload_data, files)