iOS 10: How to debug a Today Widget - "Unable to load" message
You can debug you Today Extension by doing the following:
With the application running in your device/simulator, open the widget. Then, when you have your widget open, go to Xcode->Debug->Attach to Process and you should find your widget name.
Note this will only work if you have your widget opened. You can set breakpoints and debug step by step to find out why the "Unable to Load" message appears.