chore(docs): fail on non-existing font directory

This avoids silent breakages on changes like #13093.

docs/conf.py

@@ -23,6 +23,11 @@
 # -- Path setup --------------------------------------------------------------
 
 file_dir = Path(__file__).parent.resolve()
+font_locations = (
+    "weblate/static/vendor/font-source/TTF/",
+    "weblate/static/vendor/font-kurinto/",
+)
+
 weblate_dir = file_dir.parent
 # Our extension
 sys.path.append(str(file_dir / "_ext"))
@@ -39,13 +44,15 @@ def setup(app) -> None:
         indextemplate="pair: %s; configuration value",
     )
 
-    font_dirs = [
-        str(weblate_dir / font_dir)
-        for font_dir in (
-            "weblate/static/vendor/font-source/TTF/",
-            "weblate/static/vendor/font-kurinto/",
-        )
-    ]
+    font_dirs: list[str] = []
+
+    for font_location in font_locations:
+        font_dir = weblate_dir / font_location
+        if not font_dir.is_dir():
+            msg = f"Font directory not found: {font_dir}"
+            raise NotADirectoryError(msg)
+        font_dirs.append(str(font_dir))
+
     font_files = font_manager.findSystemFonts(fontpaths=font_dirs)
 
     for font_file in font_files: