Refactor loading methods in EditorManager and OverworldEditor for improved clarity and consistency; add checks for loaded ROM in GfxGroupEditor and MessageEditor

src/app/editor/editor_manager.cc

@@ -707,7 +707,7 @@ void EditorManager::LoadAssets() {
     auto &sheet_manager = GraphicsSheetManager::GetInstance();
     ASSIGN_OR_RETURN(*sheet_manager.mutable_gfx_sheets(),
                      LoadAllGraphicsData(*rom()))
-    RETURN_IF_ERROR(overworld_editor_.LoadGraphics());
+    RETURN_IF_ERROR(overworld_editor_.Load());
     return absl::OkStatus();
   };
   if (!load_rom_assets().ok()) {