Give feedback when Publicize fails due to "Connection available to all administrators, editors, and authors" not being checked

### What

If a post is written by a non-owner, the checkbox "Connection available to all administrators, editors, and authors" must be checked in order for the post to be shared to connected social media. However, if a post fails to share for the reason, the user does not get any feedback. A message that says "Post Shared" will appear in the bottom left corner, leading the user to believe that the post was shared successfully.

Instead, a message should appear explaining that the post was not published because the author does not have permission to share and explaining how to grant such permission.

### Why

Adding an accurate message will help users to understand why posts fail to share to social media.

### How

When a post fails to share to social media because it was written by a non-owner and the checkbox "Connection available to all administrators, editors, and authors" is unchecked, replace the "Post Shared" message with another suitable message, such as "Post was not shared because the author does not have permission. Go to Tools > Marketing > Connections to manage sharing permissions."

Start at the Publicize sharing flow described in the issue and reproduce a non-owner post with the connection-availability checkbox unchecked. Confirm that the failure shows permission guidance instead of “Post Shared,” including the path to Tools > Marketing > Connections.

Written by the indexing model from the issue text.