wizard: add naive debug message to hw device scan if none are found

lib/plugins.py

@@ -461,6 +461,8 @@ class DeviceMgr(ThreadJob, PrintError):
     def unpaired_device_infos(self, handler, plugin, devices=None):
         '''Returns a list of DeviceInfo objects: one for each connected,
         unpaired device accepted by the plugin.'''
+        if not plugin.libraries_available:
+            raise Exception('Missing libraries for {}'.format(plugin.name))
         if devices is None:
             devices = self.scan_devices()
         devices = [dev for dev in devices if not self.xpub_by_id(dev.id_)]