Better error handling for Blender RPC import

- If RPC import fails, then try a direct import as well. While it's
 slower, it may be better than failing the import completely.
- Connection errors will disable RPC automatically, to avoid having to
wait the full 30 seconds timeout each time.
  This should be properly fixed by allowing to override the timeout
per HTTPClient.

modules/gltf/editor/editor_import_blend_runner.cpp

@@ -181,7 +181,18 @@ Error EditorImportBlendRunner::start_blender(const String &p_python_script, bool
 
 Error EditorImportBlendRunner::do_import(const Dictionary &p_options) {
 	if (is_using_rpc()) {
-		return do_import_rpc(p_options);
+		Error err = do_import_rpc(p_options);
+		if (err != OK) {
+			// Retry without using RPC (slow, but better than the import failing completely).
+			if (err == ERR_CONNECTION_ERROR) {
+				// Disable RPC if the connection could not be established.
+				print_error(vformat("Failed to connect to Blender via RPC, switching to direct imports of .blend files. Check your proxy and firewall settings, then RPC can be re-enabled by changing the editor setting `filesystem/import/blender/rpc_port` to %d.", rpc_port));
+				EditorSettings::get_singleton()->set_manually("filesystem/import/blender/rpc_port", 0);
+				rpc_port = 0;
+			}
+			err = do_import_direct(p_options);
+		}
+		return err;
 	} else {
 		return do_import_direct(p_options);
 	}