React component error (error) collection method
1. Add error state variables
在state中添加error变量，初始化为null。
this.state = { error : null }
2. Combine catch and setState() to capture the error object and save it in the state
fetch( url ).then( result => console.log( result ) ).catch(e => this.setState({error : e}))
3. When an error occurs, you can get the error object in the state in the render() method, and use conditional rendering to display the error message
if (error) { return <p>Something went wrong.</p> }