Add a GUI messagebox when using a Trezor but can't connect to the device.

plugins/trezor.py

@@ -133,6 +133,8 @@ class TrezorWallet(NewWallet):
                 d = HidTransport.enumerate()[0]
                 self.transport = HidTransport(d)
             except:
+                d = QDialog()
+                QMessageBox.warning(d, _('Warning'), _('Could not connect to your Trezor. Please verify the cable is connected and that no other app is using it.'), _('OK'))
                 raise Exception("Trezor not found")
             self.client = QtGuiTrezorClient(self.transport)
             self.client.set_tx_api(self)