Reduce number of fetches for posts when validating slug and add slug to err msg
Remove id from 'regular' form error message when slug already exists

- Showing the ID in this context could be confusing for the 'typical' web user
  since the interface never shows post IDs.
- Note that the API form still shows the ID in the error message because the
  API is for developers and explictily exposes post IDs to 'users' already.