Install wizard: error out when no devices found

Fixes 1668

plugins/trezor/plugin.py

@@ -220,6 +220,9 @@ class TrezorCompatiblePlugin(HW_PluginBase):
         process.  Then create the wallet accounts.'''
         devmgr = self.device_manager()
         device_info = devmgr.select_device(wallet, self)
+        if not device_info:
+            wallet.handler.show_error(_("No devices found"))
+            raise RuntimeError("No devices found")
         devmgr.pair_wallet(wallet, device_info.device.id_)
         if device_info.initialized:
             task = partial(wallet.create_hd_account, None)