统一模型字节读取失败的报错口径

- response.arrayBuffer() 原先在 fetch 的 try/catch 之外，读到一半失败会直接抛出底层 DOMException
- 改为包一层并把失败归一成 Model3dViewerLoadError 的 load-failed，与其它加载失败路径一致

packages/model3d-viewer/src/loader.ts

@@ -99,7 +99,15 @@ export async function readModel3dViewerSourceBytes(
     assertModel3dViewerByteLength(declaredLength);
   }
 
-  const data = await response.arrayBuffer();
+  let data: ArrayBuffer;
+  try {
+    data = await response.arrayBuffer();
+  } catch (error) {
+    throw new Model3dViewerLoadError(
+      'load-failed',
+      `模型数据读取失败：${describeModel3dViewerError(error)}`,
+    );
+  }
   assertModel3dViewerByteLength(data.byteLength);
   return data;
 }