Clearer error response to reject non-draft entries from Contributor

[toydi]

Currently, user with "Contributor" role can submit new drafts, but not publish new posts.

In wp-app.php, if a contributor forgets to set app:draft to 'yes' and submits a new post, the server rejects the request with an error message. But currently, the error message does not clearly mention what actually went wrong.

[Current Response]
401 Unauthorized
"Sorry, you do not have the right to edit/publish new posts."

So, here is a proposed response, which informs the contributor that: 1) you do not have the right to publish new posts, but 2) you may submit the post as draft for review.

[Proposed Response]
401 Unauthorized
"Sorry, you do not have the right to publish new posts. Please submit your post as draft for review."

[toydi]

patch with the proposed response message

[mrmist]

refreshed against 10257

[mrmist]

See also #5725

[Denis-de-Bernardy]

-1. it should autocorrect and give feedback

[Denis-de-Bernardy]

as is done in #5725