vpn: raise a clear error when no WireGuard config is configured

yt_downloader/vpn.py

@@ -62,7 +62,13 @@ def iface_name(cfg) -> str:
 
 
 def conf_path(cfg) -> Path:
-    return Path(cfg.vpn_wireguard).expanduser()
+    wg = getattr(cfg, "vpn_wireguard", None)
+    if not wg:
+        raise RuntimeError(
+            "no WireGuard config configured: set [vpn] wireguard in the config "
+            "(run 'yt-downloader init' and edit the file)"
+        )
+    return Path(wg).expanduser()
 
 
 def _host_link(iface: str) -> str: