[Feature] onError callback for ImageBlock

[johnfumaster]

Initial checklist

• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

Currently, there is no way to configure what happens when a broken image is used.

Solution

It would be useful to:

1. have an onImageLoadError config that can be passed to tag's onError
2. allow the user to config a placeholder image
3. provide a way to update the image's source

Alternatives

nothing comes to mind right now