only show quit installer option if curtin fails

and call it "Exit To Shell" instead

subiquity/controllers/installprogress.py

@@ -83,7 +83,7 @@ class InstallProgressController(BaseController):
         self.ui.set_footer(_("An error has occurred."))
         if self.progress_view is not None:
             self.progress_view.set_status(('info_error', "An error has occurred"))
-            self.progress_view.show_complete()
+            self.progress_view.show_complete(True)
         else:
             self.default()
 

subiquity/ui/views/installprogress.py

@@ -60,11 +60,14 @@ class ProgressView(BaseView):
     def set_status(self, text):
         self.linebox.set_title(text)
 
-    def show_complete(self):
-        buttons = button_pile([
+    def show_complete(self, include_exit=False):
+        buttons = [
             ok_btn(label=_("Reboot Now"), on_press=self.reboot),
-            cancel_btn(label=_("Quit Installer"), on_press=self.quit),
-            ])
+            ]
+        if include_exit:
+            buttons.append(
+                cancel_btn(label=_("Exit To Shell"), on_press=self.quit))
+        buttons = button_pile(buttons)
 
         new_focus = len(self.pile.contents)
         self.pile.contents.append((buttons, self.pile.options('pack')))