Duplicate slugs give an internal server error.

[Streammz]

Problem
When you create a new post or page in the manager, and give it a slug that already exists (either by manually filling in the slug, or using a title that results in a slug being generated that is the same as another), then attempt to save (new posts/pages) or publish (existing posts/pages) this item, it results in a 500 (Internal Server Error) response from the respective API endpoint.

Because this is an internal server error, it looks like the 'saving' progress icon in these manager buttons are forever saving. The user doesn't know about this issue unless a console is opened.

Expected behavior
The manager API should return a StatusMessage instead, as it does with other caught problems, explaining that the specified slug is already in use.

Test case

1. Open the Piranha Manager.
2. Create a page with the title Test case,
3. Save and publish this page.
4. Create another page, also with the title Test case
5. Attempt to save this second page, and watch errors appear in the browser console.