qt gui: better detection of missing PyQt5 at startup

If I use apt on ubuntu to install python3-pyqt5 and then remove it, it leaves an empty namespace:
```
$ python3
Python 3.8.10 (default, Mar 15 2022, 12:22:08)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import PyQt5
>>> import PyQt5.QtGui
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'PyQt5.QtGui'
>>> PyQt5
<module 'PyQt5' (namespace)>
```

electrum/gui/qt/__init__.py

@@ -33,6 +33,7 @@ from typing import Optional, TYPE_CHECKING, List
 
 try:
     import PyQt5
+    import PyQt5.QtGui
 except Exception:
     sys.exit("Error: Could not import PyQt5 on Linux systems, you may try 'sudo apt-get install python3-pyqt5'")