Pinterest Block: display custom message when Pinterest cannot be reached

#### Steps to reproduce the issue

1. Start with a browser using some kind of ad blocker, or add `pinterest.yourtld` to your hosts file
2. In the block editor, add a Pinterest block
3. Paste a Pinterest pin URL, such as `https://hu.pinterest.com/pin/129056345550241149/`
4. The block will be displayed as empty:


It would be nice if we could display a placeholder or something else instead.

Alternatively, you can achieve the same result with a private pin. Ideally we would display a more helpful message in those cases.

Reproduce the issue in the block editor using an ad blocker, a hosts-file entry, or a private Pinterest pin as described. Locate the Pinterest block's rendering entry point and determine how an unreachable or private pin is represented; done means the empty block is replaced by a helpful placeholder or message.

Written by the indexing model from the issue text.