init: warn, don't error, when '-upnp' is set

It prevented the GUI from starting when its settings.json had the -upnp option set. This also doesn't
prevent the node from running, so this error didn't need to be fatal.

Thanks to Sjors for bringing attention to what i broke and to Maflcko for suggestion a simple short
term fix.

src/init.cpp

@@ -561,7 +561,7 @@ void SetupServerArgs(ArgsManager& argsman, bool can_listen_ipc)
     argsman.AddArg("-peertimeout=<n>", strprintf("Specify a p2p connection timeout delay in seconds. After connecting to a peer, wait this amount of time before considering disconnection based on inactivity (minimum: 1, default: %d)", DEFAULT_PEER_CONNECT_TIMEOUT), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::CONNECTION);
     argsman.AddArg("-torcontrol=<ip>:<port>", strprintf("Tor control host and port to use if onion listening enabled (default: %s). If no port is specified, the default port of %i will be used.", DEFAULT_TOR_CONTROL, DEFAULT_TOR_CONTROL_PORT), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-torpassword=<pass>", "Tor control port password (default: empty)", ArgsManager::ALLOW_ANY | ArgsManager::SENSITIVE, OptionsCategory::CONNECTION);
-    // UPnP support was dropped. We keep `-upnp` as a hidden arg to display a more user friendly error when set. TODO: remove (here and below) for 30.0.
+    // UPnP support was dropped. We keep `-upnp` as a hidden arg to display a more user friendly error when set. TODO: remove (here and below) for 30.0. NOTE: removing this option may prevent the GUI from starting, see https://github.com/bitcoin-core/gui/issues/843.
     argsman.AddArg("-upnp", "", ArgsManager::ALLOW_ANY, OptionsCategory::HIDDEN);
     argsman.AddArg("-natpmp", strprintf("Use PCP or NAT-PMP to map the listening port (default: %u)", DEFAULT_NATPMP), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
     argsman.AddArg("-whitebind=<[permissions@]addr>", "Bind to the given address and add permission flags to the peers connecting to it. "
@@ -873,7 +873,7 @@ bool AppInitParameterInteraction(const ArgsManager& args)
     // We drop UPnP support but kept the arg as hidden for now to display a friendlier error to user who have the
     // option in their config. TODO: remove (here and above) for version 30.0.
     if (args.IsArgSet("-upnp")) {
-        return InitError(_("UPnP support was dropped in version 29.0. Consider using '-natpmp' instead."));
+        InitWarning(_("Option '-upnp' is set but UPnP support was dropped in version 29.0. Consider using '-natpmp' instead."));
     }
 
     // Error if network-specific options (-addnode, -connect, etc) are