Improve messages sent back from syndication endpoint

[paulrobertlloyd]

Describe the bug

The syndicator endpoint currently returns a 200 status (OK), but the body says this is an error, when it isn’t:

{
    "error": "OK",
    "error_description": "No posts awaiting syndication"
}

Expected behaviour

It should follow other success messages, and say something more like this:

{
    "success": "ok",
    "success_description": "No posts awaiting syndication"
}