fix: ledger old fw segwit crash

[SomberNight]

This is a follow-up of 70d23f9 and #3189

[DeviceMgr] scanning devices...
Traceback (most recent call last):
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 245, in on_hw_derivation
    xpub = self.plugin.get_xpub(device_info.device.id_, derivation, xtype, self)
  File "/home/user/wspace/electrum/plugins/ledger/ledger.py", line 488, in get_xpub
    xpub = client.get_xpub(derivation, xtype)
  File "/home/user/wspace/electrum/plugins/ledger/ledger.py", line 62, in get_xpub
    raise Exception("Firmware version too old for Segwit support. Please update at https://www.ledgerwallet.com")
Exception: Firmware version too old for Segwit support. Please update at https://www.ledgerwallet.com

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./electrum", line 373, in <module>
    d.init_gui(config, plugins)
  File "/home/user/wspace/electrum/lib/daemon.py", line 284, in init_gui
    self.gui.main()
  File "/home/user/wspace/electrum/gui/qt/__init__.py", line 248, in main
    if not self.start_new_window(path, self.config.get('url')):
  File "/home/user/wspace/electrum/gui/qt/__init__.py", line 199, in start_new_window
    wallet = wizard.run_and_get_wallet()
  File "/home/user/wspace/electrum/gui/qt/installwizard.py", line 272, in run_and_get_wallet
    self.run(action)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 60, in run
    f(*args)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 87, in new
    self.choice_dialog(title=title, message=message, choices=choices, run_next=self.on_wallet_type)
  File "/home/user/wspace/electrum/gui/qt/installwizard.py", line 91, in func_wrapper
    run_next(*out)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 105, in on_wallet_type
    self.run(action)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 60, in run
    f(*args)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 137, in choose_keystore
    self.choice_dialog(title=title, message=message, choices=choices, run_next=self.run)
  File "/home/user/wspace/electrum/gui/qt/installwizard.py", line 91, in func_wrapper
    run_next(*out)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 60, in run
    f(*args)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 215, in choose_hw_device
    self.choice_dialog(title=title, message=msg, choices=choices, run_next=self.on_device)
  File "/home/user/wspace/electrum/gui/qt/installwizard.py", line 91, in func_wrapper
    run_next(*out)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 231, in on_device
    self.derivation_dialog(f)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 239, in derivation_dialog
    self.line_dialog(run_next=f, title=_('Derivation'), message=message, default=default, test=bitcoin.is_bip32_derivation)
  File "/home/user/wspace/electrum/gui/qt/installwizard.py", line 91, in func_wrapper
    run_next(*out)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 230, in <lambda>
    f = lambda x: self.run('on_hw_derivation', name, device_info, str(x))
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 60, in run
    f(*args)
  File "/home/user/wspace/electrum/lib/base_wizard.py", line 247, in on_hw_derivation
    self.show_error(e)
  File "/home/user/wspace/electrum/gui/qt/util.py", line 190, in show_error
    _('Error'), msg)
  File "/home/user/wspace/electrum/gui/qt/util.py", line 203, in msg_box
    d = QMessageBox(icon, title, text, buttons, parent)
TypeError: arguments did not match any overloaded call:
  QMessageBox(parent: QWidget = None): argument 1 has unexpected type 'Icon'
  QMessageBox(QMessageBox.Icon, str, str, buttons: Union[QMessageBox.StandardButtons, QMessageBox.StandardButton] = QMessageBox.NoButton, parent: QWidget = None, flags: Union[Qt.WindowFlags, Qt.WindowType] = Qt.Dialog|Qt.MSWindowsFixedSizeDialogHint): argument 3 has unexpected type 'Exception'
[Plugins] stopped
[Network] stopping network
[Network] stopped
[Daemon] shutting down network
[Daemon] stopped

[coveralls]

Coverage decreased (-0.009%) to 42.764% when pulling 098c2be on SomberNight:ledger_old_fw_exception1 into f97e812 on spesmilo:master.