wizard: (trivial) add show_error to base class, document API

electrum/base_wizard.py

@@ -685,3 +685,6 @@ class BaseWizard(Logger):
             self.line_dialog(run_next=f, title=title, message=message, default='', test=lambda x: x==passphrase)
         else:
             f('')
+
+    def show_error(self, msg: Union[str, BaseException]) -> None:
+        raise NotImplementedError()

electrum/gui/kivy/main_window.py

@@ -1002,6 +1002,7 @@ class ElectrumWindow(App):
             width: width of the Bubble
             arrow_pos: arrow position for the bubble
         '''
+        text = str(text)  # so that we also handle e.g. Exception
         info_bubble = self.info_bubble
         if not info_bubble:
             info_bubble = self.info_bubble = Factory.InfoBubble()

electrum/gui/kivy/uix/dialogs/installwizard.py

@@ -2,6 +2,7 @@
 from functools import partial
 import threading
 import os
+from typing import TYPE_CHECKING
 
 from kivy.app import App
 from kivy.clock import Clock
@@ -24,6 +25,10 @@ from . import EventsDialog
 from ...i18n import _
 from .password_dialog import PasswordDialog
 
+if TYPE_CHECKING:
+    from electrum.gui.kivy.main_window import ElectrumWindow
+
+
 # global Variables
 is_test = (platform == "linux")
 test_seed = "grape impose jazz bind spatial mind jelly tourist tank today holiday stomach"
@@ -1153,7 +1158,7 @@ class InstallWizard(BaseWizard, Widget):
     def show_message(self, msg): self.show_error(msg)
 
     def show_error(self, msg):
-        app = App.get_running_app()
+        app = App.get_running_app()  # type: ElectrumWindow
         Clock.schedule_once(lambda dt: app.show_error(msg))
 
     def request_password(self, run_next, force_disable_encrypt_cb=False):