PocketBook: Run Generic:exit *last*

Makes more sense this way.

Re: #10828

Also clarify the UIManager "no dialogs left" message, and drop the
return value, as it's meaningless, we just want to break & return.

frontend/device/pocketbook/device.lua

@@ -257,18 +257,18 @@ function PocketBook:init()
 end
 
 function PocketBook:exit()
-    Generic.exit(self)
-
     -- Exit code can be shoddy on some devices due to broken library dtors calling _exit(0) from os.exit(N)
     local ko_exit = os.getenv("KO_EXIT_CODE")
     if ko_exit then
-        local fo = io.open(ko_exit, "w+")
-        if fo then
+        local f = io.open(ko_exit, "w+")
+        if f then
             -- As returned by UIManager:run() in reader.lua
-            fo:write(tostring(UIManager._exit_code))
-            fo:close()
+            f:write(tostring(UIManager._exit_code))
+            f:close()
         end
     end
+
+    Generic.exit(self)
 end
 
 function PocketBook:notifyBookState(title, document)

frontend/ui/uimanager.lua

@@ -39,6 +39,7 @@ local UIManager = {
     _refresh_func_stack = {},
     _entered_poweroff_stage = false,
     _exit_code = nil,
+    _gated_quit = nil,
     _prevent_standby_count = 0,
     _prev_prevent_standby_count = 0,
 
@@ -1448,9 +1449,9 @@ function UIManager:handleInput()
 
         -- stop when we have no window to show
         if not self._window_stack[1] then
-            logger.info("no dialog left to show")
+            logger.info("UIManager: No dialogs left to show")
             if self:_gated_quit() ~= false then
-                return nil
+                return
             end
         end