fix #2562: show error message if hardware library is missing

lib/plugins.py

@@ -392,6 +392,8 @@ class DeviceMgr(ThreadJob, PrintError):
 
     def client_for_keystore(self, plugin, handler, keystore, force_pair):
         self.print_error("getting client for keystore")
+        if handler is None:
+            raise BaseException(_("Handler not found for") + ' ' + plugin.name + '\n' + _("A library is probably missing."))
         handler.update_status(False)
         devices = self.scan_devices()
         xpub = keystore.xpub