Show info if image is no longer available (Follow-up from Resolve Images rendern)

The basic idea here is to show some information / default image that lets the user understand that the image that is supposted to be shown isn't available.

It is more a feature in order to render attachments that are links to the internet, and not so much for our own attachments. But in any case it helps the user if there is this kind of sitation.

The following discussion from !289 (merged) should be addressed:

• @nils.brinckmann started a discussion: (+1 comment)

  I found one more small UI/UX thing: If an image url can't be resolved anymore, it would be nice to have some default image / text, so that the user knows that this can't be found anymore.