qt gui: be more resilient against import issues of QtMultimedia

The user should still be able to open the Preferences dialog, despite the camera functionality being broken.

see https://github.com/spesmilo/electrum/issues/9949

```
  6.36 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "electrum\gui\qt\main_window.py", line 2672, in settings_dialog
  File "electrum\gui\qt\settings_dialog.py", line 229, in __init__
  File "electrum\gui\qt\qrreader\__init__.py", line 96, in find_system_cameras
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "PyInstaller\loader\pyimod02_importers.py", line 450, in exec_module
  File "electrum\gui\qt\qrreader\qtmultimedia\__init__.py", line 28, in <module>
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "PyInstaller\loader\pyimod02_importers.py", line 450, in exec_module
  File "electrum\gui\qt\qrreader\qtmultimedia\camera_dialog.py", line 32, in <module>
RuntimeError: PyQt6.QtMultimedia cannot import type '���d�⌂' from PyQt6.QtCore

```

electrum/gui/qt/__init__.py

@@ -44,13 +44,19 @@ from PyQt6.QtCore import QObject, pyqtSignal, QTimer, Qt
 
 import PyQt6.QtCore as QtCore
 
+from electrum.logging import Logger, get_logger
+_logger = get_logger(__name__)
+
 try:
     # Preload QtMultimedia at app start, if available.
     # We use QtMultimedia on some platforms for camera-handling, and
     # lazy-loading it later led to some crashes. Maybe due to bugs in PyQt. (see #7725)
     from PyQt6.QtMultimedia import QMediaDevices; del QMediaDevices
-except ImportError as e:
+except (ImportError, RuntimeError) as e:
+    _logger.debug(f"failed to import optional dependency: PyQt6.QtMultimedia. exc={repr(e)}")
     pass  # failure is ok; it is an optional dependency.
+else:
+    _logger.debug(f"successfully preloaded optional dependency: PyQt6.QtMultimedia")
 
 if sys.platform == "linux" and os.environ.get("APPIMAGE"):
     # For AppImage, we default to xcb qt backend, for better support of older system.
@@ -67,7 +73,6 @@ from electrum.util import (UserCancelled, profiler, send_exception_to_crash_repo
                            standardize_path)
 from electrum.wallet import Wallet, Abstract_Wallet
 from electrum.wallet_db import WalletRequiresSplit, WalletRequiresUpgrade, WalletUnfinished
-from electrum.logging import Logger
 from electrum.gui import BaseElectrumGui
 from electrum.simple_config import SimpleConfig
 from electrum.wizard import WizardViewState

electrum/gui/qt/qrreader/__init__.py

@@ -94,7 +94,8 @@ def find_system_cameras() -> Mapping[str, str]:
     if sys.platform == 'darwin' or sys.platform in ('windows', 'win32'):
         try:
             from .qtmultimedia import find_system_cameras
-        except ImportError as e:
+        except (ImportError, RuntimeError) as e:
+            _logger.exception('error importing .qtmultimedia')
             return {}
         else:
             return find_system_cameras()
@@ -143,7 +144,7 @@ def _scan_qrcode_using_qtmultimedia(
 ) -> None:
     try:
         from .qtmultimedia import QrReaderCameraDialog, CameraError
-    except ImportError as e:
+    except (ImportError, RuntimeError) as e:
         icon = QMessageBox.Icon.Warning
         title = _("QR Reader Error")
         message = _("QR reader failed to load. This may happen if "

electrum/gui/qt/qrreader/qtmultimedia/__init__.py

@@ -22,6 +22,11 @@
 # ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
+#
+# -----
+#
+# Note: This module is risky to import. At the very least, ImportError and
+#       RuntimeError needs to be handled at import time!
 
 from typing import Mapping