qt gui: input_qr_from_screenshot: better msg if screenshot is black

The input_qr_from_screenshot functionality is broken on some Linux machines:
on some machines, `screen.grabWindow(0)` returns an all-black image.

In such cases, instead of telling the user "No QR code was found on the screen",
we will tell them "Failed to take screenshot".

To test:
`QApplication.instance().primaryScreen().grabWindow(0).save("/home/user/wspace/tmp/pic2.png", "png")`

Tested on:
- machine 1:
    - ubuntu 22.04, gnome, wayland
    - pyqt.version: 5.15.9
    - qt.version: 5.15.2
    => gets all-black image for screenshot
- machine 2:
    - manjaro, kde, x11
    - pyqt.version: 5.15.9
    - qt.version: 5.15.8
    => screenshot works

I guess it might be due to x11 vs wayland.

electrum/gui/qt/util.py

@@ -585,10 +585,15 @@ class GenericInputHandler:
         if setText is None:
             setText = self.setText
         from .qrreader import scan_qr_from_image
+        screenshots = [screen.grabWindow(0).toImage()
+                       for screen in QApplication.instance().screens()]
+        if all(screen.allGray() for screen in screenshots):
+            show_error(_("Failed to take screenshot."))
+            return
         scanned_qr = None
-        for screen in QApplication.instance().screens():
+        for screenshot in screenshots:
             try:
-                scan_result = scan_qr_from_image(screen.grabWindow(0).toImage())
+                scan_result = scan_qr_from_image(screenshot)
             except MissingQrDetectionLib as e:
                 show_error(_("Unable to scan image.") + "\n" + repr(e))
                 return