Show error when proxied image cannot be loaded

[josecelano]

You can add images to the torrent description with:

![logo](https://raw.githubusercontent.com/torrust/torrust-index/develop/docs/media/torrust_logo.png)

More info: https://github.com/torrust/torrust-index-gui/blob/develop/docs/user_guide.md#upload-a-torrent.

If the proxy can get the image the web browser console shows this:

but the user can't see any error or image. We should show something.

The user can't see even the rest of the description. For example, with this description:

Ubuntu 23.04 Desktop  AMD64

![logo](https://raw.githubusercontent.com/torrust/torrust-index/develop/docs/media/torrust_logo.png)

you wouldn't see either the text "Ubuntu 23.04 Desktop AMD64" and the image.

[josecelano]

Maybe, we can show the original image URL as plain text with a message like "source image not available". But it should be clear that is not an app bug.