wizard: handle UserFacingException in WCWalletPasswordHardware

Handles `UserFacingException` in the `WCWalletPasswordHardware` step of
the hardware wallet wizard flow. This fixes the previous FIXME and
prevents the crash reporter from getting triggered if the the user e.g.
disconnects his hardware wallet during the wallet encryption step.

electrum/gui/qt/wizard/wallet.py

@@ -1275,19 +1275,40 @@ class WCWalletPasswordHardware(WalletWizardComponent):
         self.layout().addLayout(self.playout.layout())
         self.layout().addStretch(1)
 
-        self._valid = True
+        self._hw_password = None  # type: Optional[str]
+        self._valid = False
+
+    def on_ready(self):
+        _name, info = self.wizard_data['hardware_device']
+        device_id = info.device.id_
+        client = self.plugins.device_manager.client_by_id(device_id, scan_now=False)
+        if client is None:
+            self.valid = False
+            self.error = _("Client for hardware device was unpaired.")
+            return
+
+        def retrieve_password_task():
+            try:
+                self._hw_password = client.get_password_for_storage_encryption()
+                self.valid = True
+            except UserFacingException as e:
+                self.error = str(e)
+                self.valid = False
+            finally:
+                self.busy = False
+
+        self.busy = True
+        t = threading.Thread(target=retrieve_password_task, daemon=True)
+        t.start()
 
     def apply(self):
+        if not self.valid:
+            return
         self.wizard_data['encrypt'] = True
         if self.playout.should_encrypt_storage_with_xpub():
             self.wizard_data['xpub_encrypt'] = True
-            _name, _info = self.wizard_data['hardware_device']
+            assert self._hw_password
-            device_id = _info.device.id_
+            self.wizard_data['password'] = self._hw_password
-            client = self.plugins.device_manager.client_by_id(device_id, scan_now=False)
-            # client.handler = self.plugin.create_handler(self.wizard)
-            # FIXME client can be None if it was recently disconnected.
-            #       also, even if not None, this might raise (e.g. if it disconnected *just now*):
-            self.wizard_data['password'] = client.get_password_for_storage_encryption()
         else:
             self.wizard_data['xpub_encrypt'] = False
             self.wizard_data['password'] = self.playout.new_password()