Description

When trying to re-add a block which has multiple: false, the icon is disabled in the inserter, but there is no visual feedback about why. It would be nice to have some text explaining: "This block can't be added because it's already in the post/page/template" or something along these lines.

Step-by-step reproduction instructions

1. Create a post.
2. Add the More block.
3. Try to add it again.
4. Notice the button is disabled, but the user can't know why it's disabled.

That's specially confusing in the widgets screen or the site editor, where there are usually many more blocks added and some of them might be hard to find from a quick look.

Screenshots, screen recording, code snippet

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes