gui: error message if PyQt5 isn’t installed

alacritty_config_gui.py

@@ -5,7 +5,13 @@ import sys
 
 import yaml
 
+try:
     from PyQt5 import QtWidgets, QtGui, QtCore
+except:
+    print("alacritty-config-gui depends on PyQt5. I couldn’t locate PyQt5.")
+    print("Sorry about that.")
+    print("\nRunning `pip3 install PyQt5` could help.")
+    sys.exit(1)
 
 
 ALACRITTY_CONFIG = os.path.expanduser("~/.config/alacritty/alacritty.yml")