[Bug Report] Large Preview using previewSrcList does not show custom error message from slot="error"

### Element UI version
2.15.2

### OS/Browsers version
Chrome 91.0.4472.114

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/ryanjsuter/pen/eYWNEeJ

### Steps to reproduce
1. Recreated the example Image Preview example from https://element.eleme.io/#/en-US/component/image

2. Altered a link so that it was broken

### What is Expected?
Images that fail to load should display custom error message.

### What is actually happening?
Images that fail to load use a default error message that cannot be customized.