hw: allow bypassing "too old firmware" error when using hw wallets

The framework here is generic enough that it can be used for any hw plugin,
however atm only Trezor is implemented.

closes #5391

electrum/plugins/trezor/trezor.py

@@ -15,7 +15,7 @@ from electrum.logging import get_logger
 
 from ..hw_wallet import HW_PluginBase
 from ..hw_wallet.plugin import (is_any_tx_output_on_change_branch, trezor_validate_op_return_output_and_get_data,
-                                LibraryFoundButUnusable)
+                                LibraryFoundButUnusable, OutdatedHwFirmwareException)
 
 _logger = get_logger(__name__)
 
@@ -275,7 +275,7 @@ class TrezorPlugin(HW_PluginBase):
             msg = (_('Outdated {} firmware for device labelled {}. Please '
                      'download the updated firmware from {}')
                    .format(self.device, client.label(), self.firmware_URL))
-            raise UserFacingException(msg)
+            raise OutdatedHwFirmwareException(msg)
 
         # fixme: we should use: client.handler = wizard
         client.handler = self.create_handler(wizard)